Building a wall on the U.S.-Mexico border was Donald Trump's premier campaign promise, and he is running out of chances to keep it.

Trump has been president for nearly a year and a half, and has not yet won anywhere near the funding needed to build the wall along the 1,000 or so miles of border that he pledged to voters. Now, there are two Republican immigration bills in the House, each of which would provide around $25 billion for the wall, and Trump has blown them both off, saying he'd rather try again after November's midterm elections.

"Republicans should stop wasting their time on immigration until after we elect more Senators and Congressmen/women in November," Trump tweeted Friday. "Dems are just playing games, have no intention of doing anything to solve this decades old problem. We can pass great legislation after the Red Wave!"

Now, it is entirely possible Republicans will keep control of the House and Senate. It's also possible they will lose. But does anyone believe the GOP will gain enough seats to pass difficult legislation without somehow settling their internal disputes? Unlikely.

And of course Democrats are playing games with immigration. They still will be after the election, no matter what happens.

The point is, a president only gets so many chances to do something, and when it comes to the wall, Trump is running out of time.

There's no need to go through the number of times Trump promised campaign audiences that he would build the wall. He did it so much that during rallies his loyal voters developed a call-and-response on the subject. They knew that promise better than any other.

And no, the promise was not a wall along the entire 2,000 mile border between the United States and Mexico. Agreeing with a number of pro-wall experts, Trump said on many occasions that a wall was not needed along the entire stretch of sometimes-impassable land. About 1,000 miles would do the job well.

The odds were probably always against Trump on the wall. Everyone knew Democrats would throw their bodies in front of any wall effort, do anything to stop it. And even if Trump got a wall appropriation through Congress, the politically-motivated litigation designed to stop and slow the wall would be fearsome.

The only way a wall will ever happen is with determined presidential leadership. Yes, Congress has supported extensive border barriers in the past — look up the Secure Fence Act of 2006 — but the political system has never wanted to actually do it. Actually building a wall would take a president who will not give up and not settle for anything less than what he promised.

Incredibly enough, Trump made it to 2018 with a real chance to get wall funding. His maneuvers on DACA, President Barack Obama's Deferred Action for Childhood Arrivals program, set up a simple and obvious deal: legalization of DACA recipients in exchange for money for the wall.

That money, by the way, had to be real money. It couldn't be just a down payment, or a promise to come up with real cash later. It had to be real, live, appropriated, placed in a trust fund, ready for the president to use to construct a wall.

But Trump and Republicans in Congress always threw other issues into the mix. Chain migration. The visa lottery. Guest workers. Now, the hot issue of the moment is family separation. Whatever the specific add-ons, a clean DACA-for-wall deal morphed into a sort of lite version of comprehensive immigration reform, which is an endeavor that has a long record of failure in Washington.

In all of the back-and-forth, the simple clarity of the president's promise to build the wall was lost. But it will not be lost on his voters. Can Trump run for re-election in 2020 by promising to build a wall in his second term, coming up with some excuse for not doing it in his first?

He can certainly try, but even his supporters might not buy it.

Recently I texted an influential lawmaker to ask whether he thought Trump would eventually win funding for the wall. He sent back an emoji of a boy with his hands in the air in the universal how-should-I-know gesture.

That's not good enough. Trump voters expect him to do what he said, or give everything he's got trying. At the moment, the president is acting like he has all the time in the world. He doesn't.