The GOP field has rarely acknowledged any Democratic presidential candidate besides Hillary Clinton, expecting that she will be their opponent in November. That started to change Monday night.

"If Bernie Sanders or Hillary Clinton get elected -- if they were to win, we would be a great nation in decline. ... We will defeat Hillary Clinton or Bernie Sanders or whoever they nominate," Sen. Marco Rubio (R-Fla.) told supporters after his third-place finish in the Iowa caucuses. 

"We will go on to easily beat Hillary or Bernie or whoever the hell they throw up there," Donald Trump, the second-place finisher, added in his speech later in the night.

Sure, both the Republican candidates were bashing Sen. Bernie Sanders (I-Vt.). But that shows they are now seeing him as a potential competitor in the general election. Both men made their remarks as Sanders and Clinton were locked in a close battle, with the results still too close to call. 

In the last Republican debate, Sanders' name came up only briefly, in a mention by Rubio.

"It better not be Bernie Sanders. Bernie Sanders is a socialist. I think Bernie Sanders is a good candidate for president of Sweden," Rubio said. 

Clinton, meanwhile, came up over and over again -- as she always does. Republicans have consistently focused their attacks on her, even running TV ads against the former secretary of state. 

Sanders has a considerable lead in polls for the New Hampshire primary, which is taking place on Feb. 9. And if he continues to gain momentum, he will likely face more and more attacks from Republicans.

Sanders, for his part, was thrilled with the results Monday night.

"Nine months ago, we came to this beautiful state. We had no political organization, we had no money, we had no name recognition and we were taking on the most powerful political organization in the United States of America," Sanders said to a raucous crowd of supporters. "And tonight, while the results are still not known, it looks like we are in a virtual tie ... [and] we'll have about half of the Iowa delegates."