Donald Trump's prime-time speech before a joint session of Congress tonight marks a big opportunity to lay out his legislative road map to the country's future.

Unless, of course, the past gets in his way.

The presidential address has been billed by the White House as an "optimistic vision" for a safer and more economically prosperous nation, to be achieved in part by a $54-billion hike in military spending.

The president campaigned last year on promises to overhaul the tax code, boost defence, repeal the Affordable Care Act and expand immigration enforcement. If he so desires, Trump could give a unified Republican House and Senate what it probably wants to hear most — details about his plan to put "America First."

Members of the U.S. House of Representatives are sworn in on Jan. 3. Congressional scholar Mark Harkins says lawmakers on both sides aren't interested in hearing more campaign-style talk from Trump. (Jonathan Ernst/Reuters)

What legislators on both sides would most dread hearing, according to congressional scholar Mark Harkins, is "extended campaign talk" rehashing the merits of Trump's executive actions during his first month-and-a-half in office.

But things may end up going that way anyhow.

Prominent Trump spokesperson Kellyanne Conway, counsellor to the president, told Politico last week to expect a "detailed and deliberative recitation of the many things he's accomplished in the first 40 days."

A president's first joint session speech is "typically forward-looking," says Harkins, who teaches at Georgetown University's Government Affairs Institute.

"It's a table-setter" to help Congress understand a president's priorities and "the order in which he would hope Congress moves forward" to tackle them.

Harkins doesn't anticipate the president will relive the glory of his election win, or gloat about his 304 electoral college votes. But the prospect of Trump using his air time to "justify the actions of the last five-plus weeks" would "sorely disappoint Congress," he says.

It's possible Trump won't have many details to provide Tuesday about his plan to repeal and replace Obamacare. In the meantime, Republicans like Senator Chuck Grassley of Iowa are facing tough crowds in town halls. (Matt Kwong/CBC)

A successful speech could actually serve as a reset tool for Trump, redirecting chatter away from the unilateral decisions he's made since his inauguration.

Trump has generated plenty of bad press with his contentious cabinet picks such as Education Secretary Betsy DeVos, Secretary of State Rex Tillerson and Attorney General Jeff Sessions, as well as for the chaotic rollout — and subsequent defeat in court — of his executive order restricting immigration and travel to the U.S. from seven Muslim-majority countries.

'Listening for silence'

Tuesday's presidential address isn't likely to be a policy laundry list, but it could be telling if Trump leaves something out. Harkins says he'll be "listening for silence as much as for policy."

Might the president, for example, omit his pledge to introduce a $1-trillion infrastructure spending bill?

"That's fallen by the wayside a bit. Even people in Congress are talking about how that's a wonderful idea for 'next year,'" Harkins says. "This is where members of Congress are going to be looking for some priority-setting."

Legislators should expect to hear about tax reform and more about the Affordable Care Act, better known as Obamacare, though Harkins suspects the speech might not offer much more beyond platitudes about "fixing" a system Trump says is hurting Americans.

A 2009 file photo shows U.S. Representative Joe Wilson of South Carolina listening to President Barack Obama's speech about health-care reform after yelling out 'You lie!' (Jim Young/Reuters)

Republicans have been struggling to agree on a proper plan to replace the health-care law. Trump acknowledged a solution might not come until 2018.

Building up the military could be a major theme in the speech since Trump on Monday broadly outlined what he touted as a "public safety budget."

Administration officials speaking to reporters on condition of anonymity said a 10-per-cent boost to defence spending would be offset by deep cuts to most federal agencies, including the Environmental Protection Agency and the State Department. Trump has pledged the cuts won't affect Social Security and Medicare.

Best behaviour?

Although the first presidential speech to the joint session is a highly ritualized affair, and deference is to be expected for the democratically elected leader, decorum in the House was broken in September 2009. During Barack Obama's address on health-care reform, South Carolina representative Joe Wilson heckled the president, shouting "You lie!"

U.S. President Donald Trump will give his first speech to a joint session of Congress on Tuesday night. (Kevin Lamarque/Reuters)

Democrats have invited immigrants and foreign-born Muslims outraged by Trump's immigration policies to attend the joint address.

The status-quo response is for the president's party to applaud while the opposition sits on their hands. Tuesday's joint-session speech affords the president an opportunity to reach out beyond his populist base, though University of Virginia political scientist Kyle Kondik doubts that will translate to a "conciliatory" tone.

"Frankly, it doesn't seem to be his style."

Not time to 're-air grievances'

With Republicans having slim majorities in both legislative chambers, Trump will need some buy-in from across the aisle, says David Azerrad, a national policy analyst with the conservative think-tank the Heritage Foundation.

"He needs to put forward a unifying, bipartisan message of how Congress and the president are going to work together to make life better for all Americans."

Trump used the recent Conservative Political Action Conference in Maryland to slam "fake news media" as "the enemy of the people," and has railed against Senate Democrats for slowing the confirmation of his cabinet picks.

Azerrad hopes Tuesday will "not be the time to re-air grievances, but to advance the agenda with a more granular view."

"When historians write the history of the Trump presidency, the agenda is really going to get started once Congress gets involved," he says. "Now is the time to show how he's willing to work with them."