WASHINGTON — President Donald Trump called for voters to “unite behind our proud and righteous destiny as Americans” by supporting Republicans in Tuesday’s election, a final attempt to energize his base amid signs of trouble for some GOP candidates.

Trump was upbeat Monday afternoon as he addressed the first of three final rallies in Ohio, Indiana and Missouri on the eve of the midterms. He has used apocalyptic rhetoric in recent days to describe what’s at stake for the country.

“We are going to work, we are going to fight, we are going to win, win, win,” he told supporters in Cleveland.

The final three-state swing focused on states that will be key to the new Senate majority. Trump has acknowledged that Democrats could win the House, where they need a net gain of 23 seats. GOP officials and strategists voiced cautious optimism Monday about keeping the Senate, where Democrats need a net gain of two seats.

At stake Tuesday is control of Congress, 36 governorships and hundreds of down-ballot races nationwide.

Earlier Monday, Trump framed the vote as a referendum on his presidency so far, pointing to several accomplishments but saying “it’s all fragile.”

“In a certain way, I am on the ballot,” Trump said in a phone call with supporters. “Whether we consider it or not, the press is very much considering it a referendum on me and us as a movement.”

Some GOP strategists said they wished Trump would talk more about the booming economy. But in recent weeks, the president has focused his messaging on illegal immigration, the caravan of roughly 3,000 to 4,000 migrants making its way toward the U.S.-Mexico border and the possibility of voter fraud, though there is little evidence that it exists.

At a Northern Virginia campaign rally, former president Barack Obama said the election will define the soul of America, as other Democrats accused Trump of fearmongering.

“The character of this country is on the ballot,” Obama said, his voice hoarse from a flurry of campaign stops around the country. “The politics we expect is on the ballot. How we conduct ourselves in public life is on the ballot.”

Both parties spent Monday closely monitoring the handful of battleground races that will determine control of the House and Senate.

In the battle for the upper chamber, GOP operatives watched nervously amid signs of a tightening race between incumbent Missouri Sen. Claire McCaskill, a Democrat, and state attorney general Josh Hawley, a Republican.

Trump was scheduled to hold his final rally Monday night in Cape Girardeau, Mo., in hopes of giving Hawley one more boost. With no early voting option in the state, the outcome will be decided by how voters feel Tuesday.

“The president doesn’t engage in races that are surefire wins or surefire losses,” one person familiar with Trump’s political strategy said late last week, asking for anonymity because they were not authorized to speak on the record. “That race is right in between ... That race is neck and neck.”

Republican leaders have been confident in flipping a Senate seat in North Dakota, where GOP Rep. Kevin Cramer has led incumbent Democratic Sen. Heidi Heitkamp in polling.

But there was far less certainty about the likely outcomes in other competitive contests. GOP operatives noted that the broader political mood in some battleground races seemed to be moving in Democrats’ favor, at least marginally.

Several polls showed Democrats holding an edge, at least in their bid to retake the House.

A Washington Post-ABC News poll released Sunday showed that 50 percent of registered voters prefer Democratic House candidates, compared with 43 percent for Republicans.

An NBC News/Wall Street Journal poll, also released Sunday, gave Democrats a 7-point advantage on the question of which party should control the next Congress. It showed Democrats leading among women, a pivotal voting bloc this cycle, 55 percent to 37 percent.

The Justice Department announced Monday that it would deploy personnel to 35 jurisdictions in 19 states to “monitor compliance with voting rights laws,” a move that was viewed with suspicion and alarm by voting rights advocates.

Trump tweeted that law enforcement has been “strongly notified” to watch for illegal voting and that anyone “caught” would be subject to the “Maximum Criminal Penalties allowed by law.”

“Given Trump’s past animus toward immigrants and minorities, it’s certainly not a stretch to think these warnings about illegal voting along with DOJ’s announcement could be intimidating to voters and scare them from the polls,” said Sophia Lin Lakin, an attorney with the ACLU’s Voting Rights Project.

On immigration, Trump has been equally aggressive, calling the migrant caravan an “invasion.”