President Donald Trump listens to a question as he meets with former hostage Danny Burch, an oil engineer who was taken hostage in Yemen in September 2017, in the Oval Office at the White House in Washington, March 6, 2019.

U.S. President Donald Trump told Democratic leaders on Tuesday to pass the United States-Mexico-Canada trade pact before working on an infrastructure bill, a sign that crumbling roads and bridges are unlikely to get significant federal funding for repairs this year.

Trump's letter to House Speaker Nancy Pelosi and top Senate Democrat Chuck Schumer comes before a scheduled meeting at the White House on Wednesday where Democrats were expected to detail how they would like to spend $2 trillion on infrastructure improvements.

Tensions are rising between the Republican president and Democrats who on Tuesday subpoenaed two more former White House aides in connection with a House committee's probe of whether the president obstructed Robert Mueller's inquiry into Russian interference in the 2016 U.S. election.

"Before we get to infrastructure, it is my strong view that Congress should first pass the important and popular USMCA trade deal," Trump wrote.

Trump's administration negotiated a trade pact to replace the North American Free Trade Agreement with Canada and Mexico, but has yet to get Congress to approve it.

Last week, his administration announced it would remove U.S. tariffs on Canadian and Mexican steel and aluminum, a major hurdle to the passage of the agreement, but a number of Democrats have expressed concerns about other parts of the deal.

"Once Congress has passed USMCA, we should turn our attention to a bipartisan infrastructure package," Trump wrote.

Spokesmen for Schumer and Pelosi did not immediately comment on Trump's letter.

Even before Trump's letter, any infrastructure bill already faced a tough road to approval as the White House has not agreed to raise taxes or enact new taxes to fund repairs. Trump waited until 2018 to outline an initial proposal that did not include new tax revenue. The proposal largely relied on private sector and state funding, and the plan was widely panned and never got a vote in the Republican-led Congress.

Trump met with Democratic lawmakers in April, where the group agreed to spend $2 trillion to repair and build the United States' aging roads, bridges, power grids, water and broadband infrastructure, but did not develop a plan on how to pay for such a package.

Trump suggested that Congress should use the surface transportation bill as "the best vehicle to achieve our goals" on infrastructure. The existing surface transportation law expires in September 2020 and Congress has added $140 billion from the general fund to make up for shortfalls in the highway trust fund over the last decade.

That September 2020 deadline makes it less likely Congress will act on infrastructure this year, two administration officials said Tuesday.

Trump's letter said congressional Democrats canceled a scheduled meeting of their aides "preventing them from advancing our discussions. Nevertheless, I remain committed to passing an infrastructure bill."