Washington (CNN) -- Republican presidential candidate Mitt Romney picked up two more big endorsements this week, but neither appears likely to have much impact on sealing the deal for the GOP front-runner.
Tea party favorite Sen. Marco Rubio of Florida, often touted as a top possibility to be No. 2 on a GOP ticket, endorsed Romney on Wednesday, telling Fox News, "It's evidently and increasingly clear that Mitt Romney is going to become the Republican nominee. We have to come together behind who I think has earned the nomination, and that's Mitt Romney."
While Rubio said the GOP primary process has been "very good," and the candidates "have a lot to be proud of," he said having the race undecided going into the Republican National Convention in August would hurt the party in the general election.
"I think we are at a stage where two of the candidates have openly admitted the only way they can win a nomination is to have a floor fight in Tampa in August," Rubio said. "I don't think there is anything good about that. There is no way that a floor fight at the convention in Tampa in August is a recipe for a win in November. On the contrary, I think it's a recipe for disaster."
Romney also traveled to Houston on Thursday afternoon to pick up the endorsement of former President George H.W. Bush.
After a private meeting at Bush's office, the 41st president said Romney should emerge as the victor after "a very good fight" waged by some of his competitors.
"It's time when to hold 'em and time when to fold 'em," Bush said, quoting Texas crooner Kenny Rogers. "I think it's time for people to all get behind this good man."
The former president's endorsement is no surprise; the Bush family signaled its support for Romney as early as December, when George H.W. Bush told the Houston Chronicle he intended to support Romney. Former first lady Barbara Bush made robocalls for Romney in Ohio in the days leading up to Super Tuesday.
The Bushes' son, former Florida Gov. Jeb Bush, endorsed Romney last week and endorsed Rubio as a good vice presidential pick.
Rubio has said in the past that he will not appear on the 2012 Republican ticket, and on Thursday, he said his plans haven't changed.
Asked whether that Shermanesque-sounding "under no circumstances" response that he gave earlier this year still applied, Rubio said, "Yeah, I'm not going to be the vice president."
Whether these Romney endorsements will help bring the nominating contests to an end is unlikely.
CNN senior political analyst Gloria Borger said having establishment figures like Bush back Romney's candidacy doesn't help him with some of the rank-and-file.
"Within the Republican Party in particular, the power now is with the grass-roots. The power does not reside in Washington," she said. "Washington power brokers have been completely discredited because of the amount of deficit spending, for example, that occurs in Washington."
"So when you take a look at the establishment, the establishment really cannot deliver anymore because the rank and file doesn't follow anymore," Borger added.
"So there is no sort of smoke-filled room into which these people can go and come out with a solution. It is not happening and it's not going to happen. And that's why Mitt Romney has had such a hard time locking it up, because goodness knows he's got the establishment rooting for him."
A CNN/ORC International Poll released Tuesday showed most Republicans would like to see Newt Gingrich and Ron Paul end their White House bids, but favor Rick Santorum staying in to present a conservative challenge to Romney.
Gingrich and Romney held a private meeting Saturday in Louisiana that was pleasant and productive, sources told CNN's John King.
Four days later, the former House Speaker's communications director said Gingrich is determined to stay in the race for the Republican nomination -- even though he has laid off one-third of his staff and faces increasing calls to cede the contest to Romney.
Joe DeSantis said Gingrich's decision to lay off staff and replace his campaign manager was a reorganization that would enable him to fight on to the Republican National Convention in the summer and win the nomination there.
DeSantis said the poll "showed that Gingrich dropping out of the race would help Mitt Romney dramatically more than it helps Rick Santorum."
This would "virtually guarantee Mitt Romney the nomination," he said, as he urged conservatives to rally behind Gingrich to keep the former Massachusetts governor from victory.
According to CNN's latest estimate, Romney has secured 569 delegates and needs 575 more to reach the 1,144 required to clinch the GOP nomination.
Santorum, the former Pennsylvania senator, has 262 delegates, with Gingrich at 136 and Paul at 71.
Meanwhile, a big Gingrich backer said the former House speaker was nearing the end.
"It appears as though he's at the end of his line," casino mogul Sheldon Adelson said in a video posted on YouTube by the Jewish Journal on Wednesday. "Because I mean mathematically, he can't get anywhere near the numbers and is unlikely to be a brokered convention."
Adelson, who along with other members of his family channeled contributions into the pro-Gingrich super PAC Winning Our Future over the last few months, said Gingrich is the only GOP candidate in the race who is a "decision maker."
He took issue with Santorum's history as a lawmaker, saying he has no experience "creating anything or taking risks."
And he compared Romney's past to that of President Barack Obama.
"He's not a bold decision maker like Newt Gingrich is," Adelson said of Romney. "Every time I talk to him, he says 'well let me think about it.' ... He's like Obama."
CNN's Jim Acosta, John Helton, Gabriella Schwarz and Gregory Wallace contributed to this report.