Sen. Tom Cotton (R-Ark.). (Alex Brandon/AP)

Sen. Tom Cotton (R-Ark.) took a page out of President Trump's playbook Thursday morning, taking to Twitter to set the tone for the day in Washington.

But his message was decidedly counter to Trump's wishes.

After House Republicans — in the wee hours of Thursday morning — passed their Obamacare replacement through committee, Cotton wielded a giant metaphorical stop sign — before 6 a.m.

And in this case, the messenger matters as much or more than the actual message. Although this kind of caution would perhaps be expected from Sen. Lindsey O. Graham (R-S.C.), Sen. Ben Sasse (R-Neb.) or Sen. John McCain (R-Ariz.) — all three of whom have been willing to publicly break with their party's president — Cotton has gained a reputation as one of Trump's staunchest allies in the Senate.

Cotton was a vocal supporter of Trump's travel ban when most Republican senators were at least taking issue with its implementation. Back when we were tallying GOP reactions to the ban, Cotton was among just five Republican senators who offered their unqualified support. Fifteen GOP senators were more cautious, while eight outright opposed it.

“I doubt many Arkansans or Americans more broadly object to taking a harder look at foreigners coming into our country from war-torn nations with known terror networks; I think they’re wondering why we don't do that already,” Cotton said at the time.

Republicans forged ahead March 9 with their plan for a significant Obamacare overhaul despite Democratic concerns that the cost of the bill and its impact on the budget remain unknown. (Reuters)

Cotton has also defended some of Trump's more controversial positions on waterboarding and a border wall, along with Trump's targeting of Nordstrom for dropping Ivanka Trump's line and his CIA speech in which Trump boasted about his 2016 electoral victory and his crowd sizes.

Back during the 2016 campaign, Cotton departed from some of his most prominent allies and backers, such as Bill Kristol and billionaire financier Paul Singer, who tried to stop Trump at all costs. Conservative foreign policy hawks, of which Cotton is certainly one, were almost universally very wary of Trump, but Cotton stood out for not joining in their calls.

And that posture has made Cotton someone the White House leans on. RealClearPolitics last month labeled Cotton as Trump's “wingman”:

In search of a new national security adviser after Michael Flynn resigned his post earlier this month, President Trump and his inner circle leaned on input from an informal, but increasingly influential, adviser on the outside: Sen. Tom Cotton.

The Arkansas Republican would come to play a leading role at this high-stakes juncture in steering Trump toward Flynn’s replacement, Lt. Gen. H.R. McMaster — consecrating a fruitful partnership between Cotton and the White House that both sides have cultivated for months.

The piece also quoted an anonymous senior White House official who said that Cotton “always gives sound advice” and that “the president respects his judgment.”

Which brings us back to the House GOP's Obamacare replacement bill. This isn't Trump's proposal, necessarily — and the White House has shunned the “Trumpcare” label — but Trump has expressed his support for it and is reportedly working behind the scenes to pass it. The White House has said this is the bill he “ran on” — a clear signal that Republicans who oppose it are opposing him.

Plenty of Republicans not named Tom Cotton have expressed their reservations about the bill, but Cotton is now cashing in on the goodwill he has accrued with Trump and his team. And it's something that he signaled last summer.

“If Donald Trump is elected president, I will support him when he is right and we’ll try to change direction when he is wrong,” Cotton said at the Aspen Ideas Festival.

The fact that he has been more willing to vouch for Trump when he's thought Trump was right should make Cotton's words urging caution ring truer to the White House — and House Republicans.

After House Republicans released a proposal to transform the Affordable Care Act, members of their own party offered a mixture of support and suggestions for changes to the plan. (Sarah Parnass/The Washington Post)