The Republican race for the White House is tightening ahead of pivotal clashes next week in Florida and Ohio.

Donald Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE remains the front-runner and is poised for a good night on Tuesday, with polls showing him favored to win three of the four GOP contests on tap, in Michigan, Mississippi and Idaho.

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Trump has seen his leads narrow over rivals in Michigan, Ohio and Florida, however, making it clear the race for the GOP nomination isn’t over after a weekend in which Sen. Ted Cruz Rafael (Ted) Edward CruzLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list 'Parks and Rec' cast members hosting special reunion to raise money for Wisconsin Democrats MORE (R-Texas) gained on Trump in total delegates.

“If Trump comes first in Michigan, then the weekend was a detour,” former Speaker Newt Gingrich (R-Ga.) told The Hill in a phone interview. “But if you’ve got the same late deciders working against him, and Kasich comes in first [in Michigan] … then you’re in a different game.”

In Michigan, the biggest prize on Tuesday, Ohio Gov. John Kasich is making a late charge.

Trump’s lead was down to 13 points in a poll released Monday by Monmouth University that found the businessman getting 36 percent support compared to 23 percent for Cruz and 21 percent for Kasich.

The final two days of polling found Trump winning 32 percent, compared to 26 percent for Kasich and 25 percent for Cruz.

In Ohio, a survey by the left-leaning Public Policy Polling showed Trump leading Kasich by just 3 percentage points ahead of the March 15 winner-take-all primary.

And in Florida, Trump saw his lead over Sen. Marco Rubio Marco Antonio RubioFlorida senators pushing to keep Daylight Savings Time during pandemic Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Intelligence chief says Congress will get some in-person election security briefings MORE (R-Fla.) narrow to just 8 percentage points, 38 percent to 30 percent, in a Monmouth poll. Trump held 20-point leads in polls just last month in the Sunshine State, which hosts its own winner-take-all primary on March 15.

Trump maintains a lead of nearly 100 delegates over Cruz but has been the target of millions of dollars in attack ads amid an accelerating last-ditch campaign by the Republican donor class to destroy him before he can win Ohio and Florida.

Those sharpening attacks, coupled with a trend of late-deciding voters breaking against the billionaire — late-deciders in Louisiana over the weekend almost handed Cruz an upset victory — suggests Trump’s path to the nomination is narrower than it once was.

Cruz won caucuses in Kansas and Maine, and was only narrowly defeated by Trump in Louisiana’s primary and Kentucky’s caucuses. Cruz now has 300 delegates compared to Trump’s 384.

Senior Republicans caution that it’s too early to say that Trump’s trajectory has changed after the weekend’s results, but the situation will clear to a large extent over the coming week.

“A lot depends on next week,” Gingrich added. “If Kasich wins Ohio and Rubio wins Florida, we are almost certainly going towards a contested convention.”

A candidate needs to win 1,237 delegates to clinch the GOP nomination ahead of the party’s national convention in July.

Trump could still get to that number but would have a much tougher time if he loses Ohio and Florida.

And victories by Kasich and Rubio would likely keep them in the race as it heads to a winner-take-all contest in Arizona on March 22.

If no candidate wins 1,237 delegates, the race goes to the convention and a series of ballots.

Veteran Republican strategist Charlie Black believes the recent and belated wave of attack ads against Trump might be driving down the billionaire’s numbers. The anti-Trump carpet-bombing — led by groups such as the Club for Growth, Our Principles PAC, the pro-Rubio super-PAC Conservative Solutions and the nonprofit American Future Fund — is now happening in several states across the country and most intensely in Florida’s expensive media markets. The attacks will only intensify leading into next Tuesday.

Billionaire GOP mega-donors such as New York hedge fund magnate Paul Singer and the Ricketts family, which owns the Chicago Cubs baseball team, are driving these anti-Trump efforts, and new money is pouring in daily, sources close to the super-PACs have told The Hill.

The central question, says Republican strategist Ron Bonjean, is “will the millions of dollars of [anti-Trump] advertising actually make a dent with the angry white voter?”

GOP strategists interviewed for this story agreed that if the billionaire wins Tuesday’s Michigan primary and reasserts his dominance over the field, the weekend’s losses to Cruz will be seen as insignificant.

But if current polling momentum continues in Florida and Ohio, Trump could be in serious trouble.

“I think Trump is hitting a little bit of a speed bump,” Republican strategist Ford O’Connell said in a telephone interview with The Hill on Monday.

“It’s not devastating, and it can be fixed, but he’s got to go out there and put up some W’s in Florida, Michigan and Ohio.”

“He’s losing steam, but he’s still the person who as of today is most likely to go to Cleveland with the most delegates” but not necessarily 1,237, added O’Connell, who worked on John McCain John Sidney McCainThe electoral reality that the media ignores Kelly's lead widens to 10 points in Arizona Senate race: poll COVID response shows a way forward on private gun sale checks MORE’s 2008 presidential campaign but remains neutral in the current race.

The Monmouth poll suggests Trump remains a favorite to win Florida.

Rubio leads the front-runner among the nearly 1 in 5 voters who have already cast their ballots, 48 percent to 23 percent. Trump leads among those who haven’t voted yet, however, 42 percent to 26 percent.

In a hypothetical head-to-head vote between Trump and Rubio in the state, Trump still edges Rubio, taking 47 percent of the vote to Rubio’s 45 percent, according to the poll.

The poll was conducted by phone from March 3 to 6 among 403 Florida residents likely to vote in the Republican presidential primary. There is a margin of error of 4.9 points.