Montgomery County Del. Luiz Simmons, who grew up as a Republican, was elected as a Republican and this fall lost to a Republican, has decided to become a Democrat.

"I'm changing my party affiliation, not because I've changed, but because I think the Republican Party has changed," Simmons said yesterday. "The Republican Party which I grew up in, which I have been enamored of, has basically ceased to exist. The moderate wing of the party -- the Rockefeller wing when I was growing up -- has been routed by the conservatives, the Goldwaters, Helms's and Reagans."

Simmons, 33, was elected to the House of Delegates in 1978. He passed up a chance for reelection to the legislature this year to run for county executive, losing the GOP primary to political newcomer Joseph C. McGrath. Simmons blamed his defeat on a perception among conservatives that he was anti-Reagan and therefore dangerous.

"The people who worked to beat me would have supported Joe McGrath or Joe Schmo, anybody," Simmons said. "They couldn't bear the thought of supporting anybody who might say that the Republican Party should open itself up to new kinds of people. Their way of reaching out is to find a black who thinks like them or a Chicano who thinks like them. They're not interested in anybody who thinks differently, only people who look different but think the same."

State Sen. Howard A. Denis, a fellow moderate, reacted angrily to Simmons' criticisms of the party.

"This lacks credibility, it's sour grapes," Denis said. "He thinks he's going into the garden of Eden, but he'll find it's a jungle on the other side. Lou's defeat had nothing to do with the issues he's talking about. On the day he lost, Elizabeth Spencer a moderate was nominated for Congress."

"Over the years, people in this county like Gilbert Gude, Newton Steers, myself, [Del.] Connie Morella and Lou have been nominated overwhelmingly against opposition Lou would now like to classify as the majority," Denis said. "The fault, dear Brutus, is not in our stars but in ourselves. It pains me that a friend, a comrade in arms like Lou, would run away like a spoiled kid. He should sit down and analyze his own mistakes and adjust his tactics a little bit. Then he'd be just fine."

Among those Simmons consulted before making his decision was House Speaker Benjamin L. Cardin, a Democrat, who encouraged him to make the move.

"I told him I thought he was a good Democrat," Cardin said. "His views are more reflective of our party, anyway. I welcome him into the party."

One Montgomery Democrat to whom Simmons talked was senator-elect Stewart Bainum Jr.

"I told him I thought he had been an excellent legislator and he had a great record, but it could have been even better if he was a Democrat," Bainum said. "It will take him some time and hard work to gain acceptance within the party, but Lou is a hard worker so I don't think that'll be a problem."

"In some ways," Bainum added, "he has an advantage because he's never been involved in any intra-party struggles."

"I don't expect the Democrats to be knocking my door down or anything like that," Simmons said. "I expect to do scut work for a while, and I will. But I look at this as a long-range decision. I think to rebuild the Republican Party in the moderate tradition would be a lifetime's work, with uncertain results. If my heart were still in the Republican Party, I would consider it, but it's not. I would rather change parties than change my principles, and I think that's my choice.

"I do this feeling a sense of failure, but I honestly believe that the Reagan wing of the party is going to dominate it long after he is gone and I don't belong in that party. This is not a move to benefit me tomorrow or the next day, it's just a move I feel I have to make."

Simmons said that he began thinking of a party change during the primary campaign, as he became increasingly aware of the precarious position he had put himself in by questioning administration policies in his announcement speech and because of his support for a bill that would have removed country clubs' tax exemptions.

"I made a speech at a campaign forum at The Republican Men's Club, pointing out some of the dangers in the president's budget cuts and I got booed for it," he said. "These people just didn't want to hear anything critical of the President no matter who his policies might hurt."