By Robert Romano

With majorities in the House and Senate, plus President Donald Trump in the White House, the Republican Party is poised to either make big gains on signature campaign promises — reforming health care, the tax code and building the wall — or fail amid interparty squabbles over the details.

The opportunity is vast. Republicans have an advantage in 2018 in House races. They controlled a majority of state houses when they swept the 2010 elections, allowing them to redraw Congressional districts after the census was conducted.

The Senate environment is also extremely favorable to Republicans in 2018. The only seat they hold that might be difficult to defend is in Nevada. In the meantime, nine seats up for election were in states that Trump carried in 2016: Florida, Indiana, Missouri, Montana, West Virginia, Wisconsin, Michigan, Pennsylvania and Ohio.

If all those seats went Republican, even if Nevada was lost, the GOP would have a 60-vote majority, its first filibuster-proof majority since the advent of Rule XXII in 1917.

The greatest advantage however is this. Republicans control the floor of both chambers, and can move legislation to implement the agenda they campaigned on — and count on the President to sign those bills into law.

This type of opportunity is exceedingly rare for Republicans in modern history. Besides now, since the Great Depression, the GOP has only held the House, Senate and White House in 1953-1954, with Vice President Richard Nixon as a tie-breaker in the Senate under President Dwight D. Eisenhower and in 2003-2006 under President George W. Bush.

That’s it. 6 out of 84 years. Otherwise, Republican presidents have largely had to deal with mostly Democrat Congresses, making purely Republican legislative accomplishments during that period really hard to find.

Democrats on the other hand had the trifecta from 1933-1946 under Presidents Franklin Roosevelt and Harry Truman, 1949-1952 under Truman, 1961-1968 under Presidents John Kennedy and Lyndon Johnson, 1977-1980 under President Jimmy Carter, 1993-1994 under President Bill Clinton and 2009-2010 under President Barack Obama. 34 out of 84 years.

Notably, every single Democrat President since Roosevelt has enjoyed legislative majorities in both houses of Congress upon assuming office. These are the majorities that cemented the New Deal, Great Society and the Carter, Clinton and Obama economic programs that built the welfare and entitlement states. That gave us Obamacare.

This puts President Trump in very rare company among Republican presidents — but also House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell. This is a once in a generation opportunity to ratify the GOP agenda. For once, they are in control of their own destiny.

Therefore, now is the time for the GOP to prove to the American people that they made the right decision in 2016 in giving the GOP complete control of the elected branches of government — by enacting the agenda that they and President Trump promised on health care, taxes and the wall.

It could be the difference between expanding the majority — or losing it.

The headwinds Republicans face in 2018 may not be on the Senate side, but in the House. The White House incumbent party tends to lose House seats in midterm elections 89 percent of the time dating back a century, with losses averaging 35 seats. The exceptions were 1934, 1998 and 2002. That’s it.

One sure-fire way to lose the majority (and fail in this once in a century opportunity to secure a Senate supermajority) would be to dispirit Trump supporters and other Republican voters — by failing to do what they said they would do on the campaign trail.

Failure to replace Obamacare, cut taxes and build the wall will be a death knell as voters sit on their hands in 2018.

All of which makes the upcoming vote in the Senate on replacing Obamacare incredibly consequential for Republicans. Even more so since under current Congressional budgeting rules, tax cuts — the second leg of the GOP legislative strategy — are scored as adding to the deficit, and budget reconciliation measures need to be deficit-neutral.

Meaning, the larger the spending cuts that are achieved by replacing Obamacare, the greater the tax cuts will be when the budget bill is put together.

But, if Congress does not put the health care bill on the President’s desk, even an imperfect one, then doing tax cuts on budget reconciliation this year becomes that much more difficult — if not impossible. The two bills’ fates this year are linked inexorably together.

And then there’s the wall, which both McConnell and Ryan have promised will get done in the continuing resolution due Sept. 30 — which could be the key to getting Trump supporters, many of whom may not have voted in previous midterm elections, to the polls on behalf of GOP candidates in Nov. 2018.

The biggest mistake Congressional leaders could make at the moment is to underestimate the President’s support among his own supporters or to believe that they would command similar loyalty — by failing to enact the Trump agenda on health care, taxes and the wall. Now is the time to govern, because they may not get another chance in 2019.

Robert Romano is the senior editor of Americans for Limited Government.