September may be the cruelest month for House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell.

The grand (and so far failed) experiment of single-party Republican governance is about to face its toughest test yet. After seven months with no major legislative accomplishments, when lawmakers return from recess in September, they will need to grapple with raising the debt ceiling and take up the budget for fiscal year 2018, perhaps the most basic responsibility the legislature has. To pass its agenda, Republicans will need to attract enough Democrats in the Senate to sign onto their budget priorities. But the bigger question may be whether the GOP can keep itself unified enough to pass anything at all.

So far, 2017 has been marked by disunity among Republican leadership, particularly spurred by President Trump, which has resulted in legislative flame-outs. Republicans' first agenda item — to repeal ObamaCare — blew up after it became apparent that rather than have a ready replacement plan with unified caucus support, Republicans had little idea how to accomplish this seven-year pledge.

Thanks to the reconciliation process, the GOP only needed a simple majority in both chambers of Congress to get the job done. Instead, a distinct lack of trust between the party's ideological cliques emerged. A health-care bill barely squeaked through Paul Ryan's House of Representatives before the effort fell short in Mitch McConnell's Senate, as GOP members bickered over support for Planned Parenthood, the extent of repeal, and the need to help low-income Americans — all issues that voters had assumed Republicans had considered during the seven years they spent promising to repeal the Affordable Care Act.

After their health-care failure, party leadership seemed more interested in attacking each other than working together. Trump criticized McConnell's work, and McConnell criticized Trump's "excessive expectations," which led to even more sniping from the White House. Now there's word that the two aren't even speaking, and that McConnell thinks Trump's presidency might already be toast.

High-functioning political parties also generally don't split over re-electing their incumbents. Arizona Sen. Jeff Flake (R), a vulnerable senator up for re-election in 2018, went after Trump in a new "manifesto" titled The Conscience of a Conservative, taking issue with Trump's tone, tactics, and demeanor, as well as some of the Trump agenda. Flake thus far has not opposed Trump's legislative agenda as introduced to Congress, playing the role of a loyal caucus member. Nevertheless, the attack led Trump to endorse Flake's primary challenger Kelli Ward on Twitter — opening up another divide with Senate Republican leadership, which is working to defeat Ward.

Republican disunity comes at a particularly difficult moment. The White House wants to push tax reform as its next big agenda item, which has already produced the same fault lines and mistrust within the ranks as ObamaCare repeal. Not only that, but the divide is likely to worsen now that the White House has decided to add to the mix a blockbuster trade on immigration, a policy area that has split the party for more than a decade.

The strategy is to cut a deal with Democrats to clear the way for funding the border wall, along with other conservative and populist immigration priorities like e-Verify and cuts to legal immigration. Democrats have pledged to obstruct any budget that includes border-wall funding — and unlike ObamaCare repeal, they have the votes to stop it under Senate rules for appropriations bills. Meanwhile, Trump is threatening to shut down the government if he doesn't get money for his border wall.

The GOP plan has little chance of attracting significant approval among Democrats without more comprehensive reform such as normalization for current illegal immigrants, especially since Democrats will want to use immigration in next year's midterms. But it has even less chance of unifying the congressional Republican caucus to generate enough support for passage, given the mistrust between the White House, party leadership, and the fractured rank and file. Adding immigration reform to this mix is akin to pouring water on a grease fire.

In the few days remaining before the end of the recess, Trump, McConnell, and Ryan really need to sit down and work out their differences. They cannot succeed without unity of purpose and direction, and their internecine fights threaten to derail even the most basic of functions.

If they don't start putting some wins on the board and soon, this era of single-party governance promises to be fleeting.