With the House's approval of a $4 trillion Senate budget proposal on Thursday, the GOP finds itself a step closer to locking in comprehensive tax reform in time for the new year.

But how big of a deal Thursday's 216-212 vote actually is in some sense depends on whom one asks. On the one hand, the GOP now finds itself closer to comprehensive tax reform than it's been in decades.

"After watching numerous failed attempts at meaningful tax reform over the past 30 years, I couldn't help but be skeptical. … Not that I thought nothing would get done, just that the efforts would take a year and would likely end up a watered down non-event," Bryce Doty, a senior vice president and senior portfolio manager at Sit Investment Associates, said in a statement Thursday. "But this morning's budget approval by the House is a game changer."

But on the other hand, Republicans still have a long road ahead in the face of firm Democratic opposition and intraparty squabbles about the best path forward for tax reform – and not much time to iron things out before the end of 2017.

"I don't see wide Democratic support at this point. I don't think there's any at this point," says Todd Simmens, a national managing partner of tax risk management at BDO accounting and consulting outfit. "I still don't see it as happening by the end of the year."

Simmens, who served as legislative counsel to the congressional Joint Committee on Taxation when temporary tax cuts were approved under former President George W. Bush in 2001, noted that, "while we have a budget resolution in both the House and the Senate now that's agreed to … I'm not sure we have 51 Republican senators just yet on the same page."

The GOP barely had enough lawmakers in the House on the same page, as 20 Republicans defected, with many voicing opposition to the repeal of state and local tax deductions expected to be a part of an eventual tax overhaul package.

"It has a $1.5 trillion hole in it. That concerns some people," Rep. Jim Renacci, R-Ohio who voted in favor of the budget, told Fox Business Network during an interview after the measure passed. "We're looking at a Senate budget that's assuming they're going to eliminate the state and local income taxes in tax reform. There were some who said no on that."

GOP lawmakers from higher-tax states like New York voiced the strongest opposition, as their constituents could end up seeing their tax burdens swell if they can no longer deduct state and local taxes from their federal contributions. Seven of nine New York Republicans and four of five from New Jersey voted against the budget and are likely to voice similar concerns as tax discussions move forward.

"While I could not support the measure before the House today, I am hopeful that we are able to find common ground as Congress moves forward to craft a tax reform bill, and I will continue to advocate for my constituents during this process," Rep. John Katko, R-N.Y., said in a statement Thursday, indicating that he objected to the portion of the budget that "explicitly endorsed the idea of eliminating the state and local tax deduction, which would disproportionately affect New York."

And even if tax reform legislation – the specifics of which are expected to be released by tax-writing committees as soon as next week – manages to clear the House, the Senate represents another animal entirely. A health care reform effort that cleared the House earlier this year by a similar 217-213 margin ultimately ground to a halt in Capitol Hill's upper chamber.

"In the news of late, we've heard of a couple of Republican Senators who have announced they will not be seeking reelection," Simmens says. "And some press has observed that they may not be shoe-ins for whatever the package might look like or in-step with other Republicans."

Indeed, Trump has maintained ongoing feuds with key Republicans like Sens. Bob Corker of Tennessee and John McCain of Arizona – that latter of whom hammered the final nail into the coffin of the GOP's first Obamacare repeal effort under Trump.

And fellow Arizona Republican Sen. Jeff Flake on Tuesday announced he would not run for reelection in 2018 and planned to spend his remaining time in Congress standing up to what he described as "the regular and casual undermining of our democratic norms and ideals" that he believes "emanates from the top of our government."

Trump met with Republican senators over lunch on Tuesday and described the affair as a "love-fest with standing ovations," fighting off suggestions of disunion within his party. But the GOP can only afford to lose three senators if Democrats opt to collectively stonewall a tax vote in the Senate. Neither Flake, McCain nor Corker has explicitly said he won't vote for a final tax reform bill – but all three have voiced public opposition to Trump and could end up impeding his legislative agenda.

"Passing a budget with space for tax reform is a necessary, but far from sufficient, first step," a group of economists at UBS Wealth Management said in a statement Thursday. "Several Republican Senators voted for the resolution but highlighted opposition to many of the proposed policies. In other words, voting for the resolution is not voting for tax reform."

UBS predicts that, rather than comprehensive and permanent tax reform passed before the end of the year, the GOP will be forced to settle for temporary personal and corporate tax cuts that, like the Bush cuts, will expire at the end of a 10-year window.

Simmens, similarly, believes some sort of tax reform package will be ironed out eventually, but he thinks lawmakers' goal of ringing in 2018 with a new tax code sounds lofty.