Now playing
02:23
Trump and Pelosi's relationship wasn't always this bad
Now playing
01:21
Lawmaker fires back at Tucker Carlson's QAnon spin
Sen. Lisa Murkowski (R-AK) asks a question at a hearing of the Senate Health, Education, Labor and Pensions Committee on September 23, 2020 in Washington, DC.
PHOTO: Alex Edelman/Pool/Getty Images
Sen. Lisa Murkowski (R-AK) asks a question at a hearing of the Senate Health, Education, Labor and Pensions Committee on September 23, 2020 in Washington, DC.
Now playing
01:40
Trump plans to campaign against Sen. Murkowski in 2022
Biden 03062021
PHOTO: CNN
Biden 03062021
Now playing
02:28
'Help is on the way': Biden speaks after Senate passes relief plan
Now playing
03:04
Schumer: Nobody said it would be easy, but it is done
01 senate stimulus bill 210306
PHOTO: Senate TV
01 senate stimulus bill 210306
Now playing
01:47
Senate passes Biden's $1.9 trillion Covid-19 relief bill
WASHINGTON, DC - MARCH 05: Sen. John Cornyn (R) (R-TX) talks with Sen. Joe Manchin (D-WV) while walking to the U.S. Senate chamber for a vote March 05, 2021 in Washington, DC. The Senate continues to debate the latest COVID-19 relief bill.  (Photo by Win McNamee/Getty Images)
PHOTO: Win McNamee/Getty Images
WASHINGTON, DC - MARCH 05: Sen. John Cornyn (R) (R-TX) talks with Sen. Joe Manchin (D-WV) while walking to the U.S. Senate chamber for a vote March 05, 2021 in Washington, DC. The Senate continues to debate the latest COVID-19 relief bill. (Photo by Win McNamee/Getty Images)
Now playing
02:54
Axelrod breaks down Manchin's surprising move
sinema
PHOTO: CNN
sinema
Now playing
01:50
Senator's move has many on the internet outraged
PHOTO: FBI
Now playing
02:58
Trump State Department official charged in Capitol riot
WASHINGTON, DC - JANUARY 06: U.S. Sen. Paul Gosar (R-AZ) speaks on the floor of the House Chamber during a joint session of congress on January 06, 2021 in Washington, DC. Congress held a joint session today to ratify President-elect Joe Biden's 306-232 Electoral College win over President Donald Trump. A group of Republican senators said they would reject the Electoral College votes of several states unless Congress appointed a commission to audit the election results. (Photo by Drew Angerer/Getty Images)
PHOTO: Drew Angerer/Getty Images
WASHINGTON, DC - JANUARY 06: U.S. Sen. Paul Gosar (R-AZ) speaks on the floor of the House Chamber during a joint session of congress on January 06, 2021 in Washington, DC. Congress held a joint session today to ratify President-elect Joe Biden's 306-232 Electoral College win over President Donald Trump. A group of Republican senators said they would reject the Electoral College votes of several states unless Congress appointed a commission to audit the election results. (Photo by Drew Angerer/Getty Images)
Now playing
02:19
This is what Rep. Gosar was posting days before Capitol riot
John King Magic Wall 0305
PHOTO: CNN
John King Magic Wall 0305
Now playing
02:17
President Biden sending a team to the US-Mexico border
Senior Advisor to the President Jared Kushner  attends a press conference on September 4, 2020, in the Brady Briefing Room of the White House in Washington, DC. (Photo by Brendan Smialowski / AFP) (Photo by BRENDAN SMIALOWSKI/AFP via Getty Images)
PHOTO: BRENDAN SMIALOWSKI/AFP/AFP via Getty Images
Senior Advisor to the President Jared Kushner attends a press conference on September 4, 2020, in the Brady Briefing Room of the White House in Washington, DC. (Photo by Brendan Smialowski / AFP) (Photo by BRENDAN SMIALOWSKI/AFP via Getty Images)
Now playing
02:50
Jared Kushner disappears from Trump's inner circle
Rep john garamendi 0305
PHOTO: CNN
Rep john garamendi 0305
Now playing
02:33
Rep. Garamendi: Any lawmaker involved in Capitol riots ought to be thrown out of Congress
Protesters gather at Lincoln Park to demand the Emancipation Memorial be taken down on June 23, 2020 in Washington, DC.
PHOTO: Tasos Katopodis/Getty Images
Protesters gather at Lincoln Park to demand the Emancipation Memorial be taken down on June 23, 2020 in Washington, DC.
Now playing
03:01
Why some people want this Abraham Lincoln statue taken down
psaki
PHOTO: CNN
psaki
Now playing
00:56
Psaki fires back at Trump testing czar over vaccine claims
PHOTO: CNN
Now playing
03:04
Avlon: Pence's op-ed is 'way worse than Stockholm syndrome'

Editor’s Note: Paul Callan is a CNN legal analyst, a former New York homicide prosecutor and of counsel to the New York law firm of Edelman & Edelman PC, focusing on wrongful conviction and civil rights cases. Follow him on Twitter @paulcallan. The opinions expressed in this commentary are his own. View more opinion on CNN.

(CNN) —  

House Democrats will press forward on Thursday with a House vote to officially greenlight impeachment proceedings into President Donald Trump’s handling of aid to Ukraine while he was seeking an investigation of his political rival, Joe Biden.

In defending his actions, Trump has suggested that reporters should look into Vice President Mike Pence’s conversations with the Ukrainian president. And the White House has been evaluating whether to release notes of Pence’s talks with President Volodymyr Zelensky.

All of these developments raise the question of whether Pence could be drawn into the impeachment maelstrom.

Should Democratic efforts to remove Trump from office succeed, Pence would succeed to the presidency – under normal circumstances.

Both history and constitutional law, however, suggest that other successors to the office are possible. It all depends on the magnitude and nature of the “bribery” or other “high crimes and misdemeanors” that may or may not be established by congressional investigators.

More importantly, presidential succession, in this case, depends on (to paraphrase former Sen. Howard Baker in reference to the Watergate Investigation) what the Vice President knew and when he knew it. Specifically, was the vice president aware during his September 1 conference with Zelensky in Poland, that Trump had previously asked for a “favor” when discussing military aid and that the favor included a Ukrainian criminal probe of Joe and Hunter Biden. The political futures of Mike Pence and Nancy Pelosi may well depend upon the answers to these questions.

If Pence’s September trip was intentionally designed to reinforce Trump’s demand for a “favor” with an intimidating follow-up message from the vice president, Pence may have a serious problem.

It could put him right in the thick of the impeachment soup as a co-conspirator with the president.

What might follow could be a double impeachment scenario that would make heads spin and House Speaker Nancy Pelosi the president of the United States. After Pence, she’s next in the line of succession.

Pence has already admitted that he visited President Zelensky and raised concerns about corruption in Ukraine in the weeks after President Trump’s controversial and alleged “quid pro quo” discussion with the newly elected Ukrainian leader. One of Pence’s aides was on the call with the Ukrainian President. The specifics of that conversation have yet to be made public but will ultimately reveal whether Pence was actively involved in an attempt to improperly pressure the vulnerable Ukrainian leader.

Some sources cited by CNN reporting claim that the Vice President’s version of his conversation with the Zelensky is that it wasn’t about Biden, but about corruption generally in Ukraine. Pence brought Trump assurances from Zelensky that the Ukrainian leader would take action against corruption. In this account, Pence then made the case to Trump that the United States should lift its hold on aid to Ukraine, which is what happened.

If Trump and Pence were both implicated in the scandal and Pence’s resignation or removal from office occurred first, Trump could nominate a new vice president under the 25th Amendment.

The vice-presidential appointment, however, would have to be approved by a majority in both the Senate and the House of Representatives to become effective. What are the chances that a Democratic majority in the House would approve any vice-presidential choice from Trump? Next to zero would be my bet – especially given that the rejection of a Trump vice-presidential choice would elevate Pelosi to the presidency in the event of Trump’s removal from office after an impeachment trial in the Senate.

A Speaker’s assumption of the presidency came perilously close to happening once before, when the Watergate scandal brought an end to the Nixon Administration in 1974.

Nixon’s vice president during the early stages of the Watergate investigation was Spiro Agnew, the former governor of Maryland. An investigation by the Justice Department revealed that as governor of Maryland, Agnew had engaged in various acts of political corruption, including bribe-taking that continued even after his assumption of the vice presidency.

At first, Agnew contested the charges, even asserting the defense that a sitting vice president is immune from indictment. That defense – which sounds suspiciously like the claim asserted by Trump lawyers before the federal Second Circuit Court of Appeals earlier this month that even if the President shot someone on Fifth Avenue, he would be immune from criminal prosecution – was quickly abandoned by Agnew.

In a negotiated deal in October of 1973, Agnew pled nolo contendere to a single felony count of income tax evasion and resigned from office. He received a slap-on-the-wrist sentence of probation and a fine.

While both Agnew and Nixon were under the cloud of possible criminal indictment and impeachment, Speaker of the House Carl Albert, a Democrat, asked former John F. Kennedy speech writer Ted Sorensen to prepare a “contingency plan” outlining the circumstances under which Albert would have to assume the presidency under the 25th Amendment.

Get our free weekly newsletter

Sorensen’s memo explained that had the vice presidency remained vacant at the time of a Nixon removal from office, Albert would have become president under the 25th Amendment.

Those plans were dashed, of course, when President Nixon immediately appointed the very popular Gerald Ford, the Republican minority House leader, as vice president after Agnew’s resignation.

The appointment was swiftly approved by a majority vote of both houses of Congress two months later. Perhaps it’s time for Pelosi to brush the dust-off Ted Sorensen’s contingency plan memo. You never know what the future may bring in DC these days.