Donald Trump is shown at a campaign rally on Monday,...

Donald Trump is shown at a campaign rally on Monday, May 2, 2016, in Carmel, Ind. Democrats were viewing his likely nomination as the GOP presidential candidate with glee, convinced that he will influence congressional races and help them gain seats. Credit: Getty Images / Joe Raedle

WASHINGTON — Democrats are viewing the increasingly likely nomination of Donald Trump as the Republican presidential candidate with glee, convinced that he will cast a long shadow over congressional races and help Democrats gain seats, if not control, of Congress.

Trump is their best bet to win the net 30 House seats — including two on Long Island that pundits have called toss ups — and net five Senate seats to take control of at least the Senate and make inroads into the Republican majority in the House, say some Democratic leaders.

“We’re making sure their field [of Republican candidates] is tied to Donald Trump and that Donald Trump is tied to them,” said Rep. Ben Lujan (D-N.M.), chairman of the Democrats’ House campaign.

Republicans deny that Trump is a sure campaign killer for their House and Senate candidates, saying it is too early to grasp the full effect of a Trump campaign, something that won’t be clear until after the July nominating conventions are over.

“We literally don’t know. We’re operating in a vacuum,” said Rep. Greg Walden (R-Ore.), who leads the GOP House campaign.

As Trump and Democratic front-runner Hillary Clinton come closer to clinching their respective party’s presidential nomination, their impact on congressional campaigns has become an issue of growing interest because the outcome could change the dynamic of Washington.

If the winner of the White House has a Congress controlled by his or her party, there could be a break in the hard-edged partisanship that has gridlocked Washington since 2011.

Lujan, the chairman of the Democratic Congressional Campaign Committee, underscored his strategy of linking Republican candidates to Trump by handing out a list of Republicans running for the House with comments they’ve made in support of the New York businessman and reality TV star.

Trump has high negatives, surveys have found.

Five polls taken last month reported that 31 percent to 33 percent of those surveyed had favorable views of Trump, but that 61 percent to 67 percent had unfavorable views of him, according the political website Real Clear Politics.

But Walden, who chairs the National Republican Congressional Committee, objected. “We haven’t talked much about Hillary Clinton,” he said. “She has some of the highest negative ratings.”

Those same polls found 31 percent to 42 percent had favorable views of Clinton, but that more than half, 54 percent to 56 percent, had unfavorable views of her.

That means if Trump and Clinton become the presidential nominees, as appears increasingly likely, it will be the first presidential election in which both candidates started with higher negative than positive ratings.

And that could lead to some unpredictability. Consider the voting in the April 19 primary in Long Island’s two “tossup” congressional districts.

In the 3rd District in Nassau, Queens and Suffolk counties, where Rep. Steve Israel (D-Huntington) is retiring, 60,713 Democrats voted, spurred in part by a hot state Senate race — and 37,730 chose Clinton. Just 40,156 Republicans voted, most of them — 30,369 — for Trump.

But in the 1st Congressional District on Long Island’s East End, where Rep. Lee Zeldin (R-Shirley) is seeking a second term, a different pattern emerged.

A total of 42,176 people voted as Democrats, and Clinton just edged out her rival, Sen. Bernie Sanders (I-Vt.), by a 1,550 vote margin, 21,861 to 20,311.

But 50,567 people voted as Republicans — and Trump won easily with 37,137 votes.