President Donald Trump ran on rebuilding the nationís roads, rails and ports, a priority that members of both parties share. Yet no additional miles of highway, no new runways, no more miles of track will be built without a substantial infusion of federal money. Agreement on whether the nation needs better infrastructure has never been elusive. How to pay for it has. On that tough question, it is time for Trump to step up.The issue is tough not because Congress lacks reasonable options for raising infrastructure money. The solution to the Highway Trust Fundís perpetual funding woes is to raise more money from the same source that has supported the fund for decades: the federal gasoline tax. This point was underscored last week in a hearing before a House transportation subcommittee, where witnesses and lawmakers alike argued for a modest boost in the gas levy, which has been stuck at 18.4 cents per gallon since 1993.Because the tax was not set to rise with inflation, its value has eroded steadily for 25 years. Gas tax revenue has now lost 40 percent of its purchasing power, a representative from the U.S. Chamber of Commerce reported. The typically tax-averse group favors a 25-cent increase. The American Trucking Associations, another group one would assume would oppose a higher gas tax, favors a 20-cent hike. "Itís imperative that this president put the full power of his office behind this," Chris Spear, the president of the trucking organization, said. "If he wants it done this year, heís going to have to lead it, not just throw everything on the table."That is because the politics of raising the gas tax are nearly impossible. The notion hits a wall of resistance among Republicans opposed to tax increases of any type, no matter how rational. Enthusiasm for a potentially unpopular hike in gasoline prices is not exactly robust among Democrats, either. House Transportation Committee Chairman Bill Shuster, R-Pa., has argued that conservatives should like the gas tax because it puts the burden of paying for the roads on those who use them rather than on the taxpayer at large. But it is unclear whether the idea has a path through the Ways and Means Committee, which would have to approve an increase. For his part, the tax-allergic House speaker, Paul Ryan, R-Wis., has long opposed a hike.That is why Trumpís intervention is crucial. Popular with the GOP base, the president could give cover to Republicans who understand the virtues of raising the gas tax but fear the political repercussions. Unlike congressional leaders, Trump is reportedly sympathetic to the effort to raise the levy by a quarter. If he is serious about advancing an infrastructure plan ó he rolled out a package last month that would require $200 billion in federal investment ó he will need the cash. The country could certainly use it.