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(CNN) —  

With the final primaries of the 2016 nomination season approaching, a new CNN Poll of Polls finds the candidates most likely to lead their parties into the general election are locked in a tight contest. Hillary Clinton holds an average of 45% support while 43% back Donald Trump across five recent nationwide polls of registered voters.

The Poll of Polls suggests a vastly different race than many anticipated.

Throughout March and April, public polling on the presidential race found Clinton well ahead of Trump, with the former secretary of state holding double-digit leads over the businessman in 10 out of the 14 polls that met CNN’s standards for reporting in those months.

When Trump hits back, he hits back hard

A string of polls released in the last two weeks, however, suggest a much tighter contest. For the most part, those tightened margins come from a more unified Republican Party. Across the five polls included in the Poll of Polls, Clinton averaged 6% support among self-identified Republicans. Polls from the same organizations in late-March and April found Clinton averaging 12% support among Republicans.

What changed? Trump sealed the deal.

Trump essentially locked up the Republican nomination for president in early May, when his final remaining rivals suspended their campaigns after Trump won the Indiana primary by a wide margin. Last week, Trump secured the support of enough unbound delegates to reach the 1,237-delegate threshold needed to win the nomination.

Clinton: Trump shamed into making donations to veterans groups

Clinton, by contrast, has not yet reached the 2,383-delegate threshold the Democratic Party requires, and rival Bernie Sanders continues his campaign. Several of these polls have found Clinton’s support in a general election matchup against Trump continues to lag among those who back Sanders, softening her numbers at the same time Trump’s are solidifying.

But a tight contest between Clinton and Trump isn’t entirely a new phenomenon. Across six national, live-interviewer telephone polls conducted between the start of the year and mid-February, Clinton led Trump by just 2 points on average, and a tight race between the two was more common than a big Clinton lead in polling from early fall of last year through this spring.

A close divide is a logical place for the race to be at this stage. Many studies have demonstrated the sharp political polarization that drives many Americans’ views on key issues. At the same time, the share of voters truly “up for grabs” in any given election cycle has dwindled. The most recent Washington Post-ABC News poll found that just 7% of registered voters who aren’t currently supporting Trump say they would consider backing him, and likewise, just 7% of those not already backing Clinton would consider supporting her.

Clinton’s flood-the-zone Trump strategy

The Poll of Polls suggests that about 1-in-8 registered voters isn’t currently backing Clinton or Trump. Over the next five months, whether those voters can be convinced to support Clinton or Trump, decide to stay home or throw their backing to a third party candidate will be the key to the campaign.

The CNN Poll of Polls is an average of the results of the five most recent nationwide polls of presidential preference among registered voters. The Poll of Polls includes: The CBS News/New York Times poll conducted May 13-17, the Fox News poll conducted May 14-17, the NBC News/Wall Street Journal poll conducted May 15-19, the ABC News/Washington Post poll conducted May 16-19, and the Quinnipiac University Poll conducted May 24-30. The poll of polls does not have a margin of sampling error.