President Donald Trump (L) waves as he arrives at the Capitol with Director of the National Economic Council Gary Cohn (R) for a House Republican Conference meeting November 16, 2017 in Washington, DC.

President Donald Trump's year-end agenda is at a critical juncture as he heads Tuesday to Capitol Hill to rally Senate Republicans on taxes — and then pivot to negotiations with Democrats pressing for victories of their own in a separate, high-stakes showdown over the budget and immigration.

Trump is still seeking his first marquee win in Congress, but the White House and top GOP leaders have work to do to get their tax bill in shape before a hoped-for vote later this week. Party deficit hawks pressed for a "backstop" mechanism to limit the risk of a spiral in the deficit, even as defenders of small business pressed for more generous treatment for Main Street.

On a separate track from taxes is a multi-layered negotiation over a huge Pentagon budget increase sought by Trump and Republicans and increases for domestic programs demanded by Democrats. Democrats carry leverage into the talks, which have GOP conservatives on edge.

A temporary spending bill expires Dec. 8 and another is needed to prevent a government shutdown. Hurricane aid weighs in the balance and Democrats are pressing for legislative protections for immigrants known as "Dreamers," even as conservative Republicans object to including the issue in the crush of year-end business.

Tuesday would bring Trump's third visit to the Capitol in little more than a month — this time to make the sale to Senate Republicans on his signature tax bill. But among the holdouts are GOP Trump critics, including Sens. Jeff Flake of Arizona and Bob Corker of Tennessee — though GOP leaders are seeking to rope in straggling Republicans with a flurry of deal-cutting.

"There's still some loose ends. We're not quite there yet," said Sen. Rob Portman, R-Ohio. "But I think we're going to get there, I really do."

Trump's sessions with big groups of Republicans tend to take the form of pep rallies, and when visiting a Senate GOP lunch last month Trump spent much of the time on a rambling account of the accomplishments of his administration.

Later on Tuesday, the bipartisan top four leaders of Congress — Speaker Paul Ryan, R-Wis., and Minority Leader Nancy Pelosi, D-Calif., for the House and Senate Majority Leader Mitch McConnell, R-Ky., and Minority Leader Chuck Schumer, D-N.Y. — will head to the White House to touch gloves on a range of year-end issues.

Topping the bipartisan agenda is a heavily-sought year-end spending package to give both the Pentagon and domestic agencies relief from a budget freeze.

Trump hasn't engaged much with Pelosi and Schumer since a September meeting that produced an agreement on a short-term increase in the government's so-called debt limit and a temporary spending bill that is keeping the government's doors open through Dec. 8.

Trump reveled in the bipartisan deal for a time and generated excitement among Democrats when he told them he would sign legislation to protect from deportation immigrants who were brought to the country illegally as children.

Trump in September reversed an executive order by former President Barack Obama that gave protections to these immigrants, many of whom have little or no connection to their home country. Shortly afterward, he told Pelosi and Schumer he would sign legislation protecting those immigrants, provided Democrats made concessions of their own on border security.

Since the president is such a wild card, neither Democrats nor Republicans were speculating much about what Tuesday's meeting might produce.

"Hopefully, we can make progress on an agreement that covers those time-sensitive issues and keeps the government running and working for the American people," Schumer said.