Republican presidential candidate Donald Trump speaks during a campaign rally in Las Vegas. (AP Photo/Isaac Brekken)

PELLA, Iowa — Republican presidential candidate Donald Trump's campaign piqued the interest of political observers when a special guest appeared at an event Saturday here in Pella: Sen. Chuck Grassley, a Hawkeye State institution and a prominent Republican voice in Washington.

“I want Mr. Trump to know that I appreciate his support for me and most importantly for Iowa being first in the nation, our all important Iowa caucus,” Grassley told the crowd of about 500 as he introduced Trump at Central College. “...We’ve got to keep up this energy that’s shown here today and many other places around Iowa because that is what it’s going to take for us to win back the White House in November."

Grassley, who did not endorse Trump, has introduced several other candidates campaigning in Iowa. But his presence at the Trump campaign event nonetheless demonstrated the increasingly warm relationship between Trump and the Republican political establishment, which was initially stunned and disheartened by the flamboyant billionaire’s enormous lead over other more establishment-friendly candidates.

Grassley, a widely popular figure in Iowa who has represented the state in the U.S. Senate for decades, gave Trump a warm introduction and talked about the importance of electing a Republican to the White House.

“We have an opportunity once again to make America great again,” Grassley told the crowd, echoing Trump's campaign slogan and urging them to vote.

Jill Kozeny, Grassley's chief of staff, told the Post that he will not endorse anyone in the Iowa caucus. "He's attended events in Iowa for all Republican presidential candidates since last summer, whenever he's been invited and it can be scheduled," she said. "He's committed to doing whatever he can to help elect a Republican president this year." Grassley will also introduce Sen. Marco Rubio (Fla,) next week during a campaign event in Iowa, according to a Rubio campaign spokesman.

[GOP establishment warms to Trump — and remains cool toward Cruz.]

But the senator’s appearance comes after four-term Iowa Gov. Terry Branstad (R) explicitly spoke out against Sen. Ted Cruz (Tex.), Trump's most competitive rival in Iowa, who has few friends in Washington and has been criticized by the political class in Iowa.

"Ted Cruz is ahead right now. What we’re trying to do is educate the people in the state of Iowa. He is the biggest opponent of renewable fuels," Branstad said Tuesday. "I think it would be a big mistake for Iowa to support him."

Trump asked Grassley to stand again after he took the stage, telling the crowd he is "a great guy."

Cruz — who is trailing Trump in Iowa with nine days to go before the Iowa caucuses — has attempted to use the thawing relationship between Trump and party leaders to undermine real estate mogul’s populist appeal. Trump has responded by criticizing Cruz for failing to work with his colleagues in the Senate.

“Here’s a United States senator, Republican, doesn't have the support of one other Republican senator,” Trump said during a ballroom speech in Las Vegas Thursday. "There’s something wrong there."

[Trump knocks Cruz on icy relationships in Congress, says deals are ‘what the country’s about’]

Fueled by a deep anti-Washington energy from the start of his unlikely presidential campaign, Trump has nonetheless indicated recently that he is open to bridging divisions between the GOP’s activist wing and party leaders in Washington. “Ronald Reagan would get along with Tip O’Neill and they’d sit down and make great deals for everybody. That’s what the country’s about,” Trump added in Las Vegas.

But he has rrejected Cruz’s criticism that such support makes him behold to the party leadership; he has taken to pointing to the high-profile endorsement by Sarah Palin Tuesday to prove that he is not an establishment figure.

"I got Sarah Palin. I'll take Sarah Palin every single day," Trump told the crowd.