On Friday, Sen. Ted Cruz (R-Texas) joined the ranks of originally reluctant Republican lawmakers to endorse the party’s presidential nominee Donald Trump. The surprise announcement is less surprising if you consider what Cruz had at stake by holding out his support.

AP/Dennis Van Tine - apimages.com

After famously declining to endorse Trump — and urging voters to “vote your conscience” — at the Republican National Convention in July, Cruz’s favorable ratings plummeted; 61 percent of Republicans viewed him favorably at his peak in January, and only 39 percent felt the same after his speech, according to Gallup:

Gallup - gallup.com

What's more, The New York Times reported that Cruz's non-endorsement cost him at least two "influential donors."

That could pose a problem for Cruz, who will be up for re-election in 2018. The senator made a gamble, betting that voters would appreciate his principled stance against the party nominee (whose favorable ratings among Republicans are also historically low compared to past GOP presidential nominees), but now he's reversed course.

Here's how Cruz framed his endorsement in a Facebook post on Friday.

"After many months of careful consideration, of prayer and searching my own conscience, I have decided that on Election Day, I will vote for the Republican nominee, Donald Trump. I’ve made this decision for two reasons. First, last year, I promised to support the Republican nominee. And I intend to keep my word. Second, even though I have had areas of significant disagreement with our nominee, by any measure Hillary Clinton is wholly unacceptable — that’s why I have always been #NeverHillary."

But that "promise" didn't keep Cruz from withholding his endorsement in July. Nor did the prospect of Democratic presidential nominee Hillary Clinton's victory hold much sway over his decision. So what changed?

"One way to think about it is, if Ted Cruz’s stand against Donald Trump was a purely principled one — based on a combination of disagreement with Trump’s position on the policies as well as because of the attacks that Trump made against Cruz’s father and wife — we might have expected him to continue to hold out and withhold an endorsement from Donald Trump," Dr. Mark Jones, a political science fellow at Rice University’s Baker Institute for Public Policy, told ATTN:. "However, Ted Cruz’s approach was also based on political calculation."

That calculation was that Trump's campaign was destined for failure — that he'd "implode and that we were looking at a Hillary Clinton landslide," Jones said.

AP/Evan Vucci - apimages.com

Now, with the race tightening ahead of the first presidential debate, Cruz was feeling the pressure. In particular, he was likely thinking ahead to the statewide Republican primary in 2018.

"By endorsing Trump today — albeit a relatively lukewarm endorsement — Cruz likely headed off any potential primary challenge in March of 2018," Jones said. "And once you win the Republican primary statewide in Texas, you’re a virtual lock in November, so he will have effectively, by his endorsement today, enhanced his odds of obtaining re-election in 2018 to somewhere in the 90-95 percent range."