Senate Democratic hopefuls are racing to link their opponents to Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE as he becomes the last man standing in the race for the GOP nomination.

Democrats quickly pounced Wednesday as Trump locked down his status as the presumptive presidential nominee after winning the Indiana primary, forcing Sen. Ted Cruz Rafael (Ted) Edward CruzSenate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week Renewed focus on Trump's Supreme Court list after Ginsburg's death MORE (R-Texas) and former Ohio Gov. John Kasich to suspend their campaigns.

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"With the Republican National Convention in Cleveland, there will be no rock that Portman can hide under to avoid his Party's toxic nominee," he added.

Portman, who supported Kasich, has said he would ultimately support whoever wins the nomination. Like other vulnerable GOP incumbents he's adamant he'll be able localize his election despite Trump's ability to dominate a media cycle.

Democrats, however, warn that Republicans can't outrun Trump's shadow.

Democratic Gov. Maggie Hassan's campaign blasted out an email titled "huh?" that included tweets from reporters questioning the distinction between "supporting" and "endorsing" a candidate.

Portman and Ayotte are at the center of the fight for the Senate. Republicans are defending 24 Senate seats in November, including a handful in state's previously carried by President Obama.

Republicans have tried to navigate a precarious path on Trump. While they've aimed to create space from the presidential race, they also can't afford to completely cut ties with the real estate mogul or his supporters.

The Democratic Senatorial Campaign Committee said Republicans will have to "explain their own out-of-touch records while running alongside Trump ... who has now taken over their party."

Democrats are optimistic that Trump as the nominee will ultimately help drag down GOP Senate candidates, particularly in swing states where they'll need to win over independent voters and moderates. To win back control of the Senate, Democrats need to flip four seats if the White House is won by a Democrat, or five seats if a Republican wins the presidency. Katie McGinty, the Democratic challenger in Pennsylvania, is showing no signs of changing her strategy, even though Sen. Pat Toomey (R-Pa.) said linking him to Trump is "outrageous."

"The Trump-Toomey ticket is official tonight. Toomey has pledged to support Trump as GOP nominee despite his hateful rhetoric," she tweeted on Tuesday evening.

Republicans have been preparing for months for the potential that Trump would ultimately win the nomination, arguing vulnerable incumbents should focus on local issues.

While Democrats need to pick up a handful of seats to win back the Senate, they hope Trump will be an albatross to the GOP even in state's considered safe Republican seats.

Eldridge doubled down Wednesday, saying Boozman should be forced to say if he still supports Trump.

"It’s the easiest softball he will get all election season: Do you still support Donald Trump as the Republican nominee after witnessing his reprehensible treatment of women?" he said in a statement.

—Lisa Hagen contributed.