After testing the political waters for more than two months, Sen. Dale Bumpers (D-Ark.) has decided to seek the Democratic presidential nomination and intends to form an exploratory campaign committee next week, political advisers said yesterday.

Bumpers, 57, has tentative plans to announce the formation of the committee, which he considers equivalent to a formal declaration of candidacy, next Wednesday, advisers said.

"He's been moving in this direction for a long time, and he finally feels comfortable in running," one longtime Arkansas ally said. "He thinks he can win."

He said Bumpers' supporters had convinced him that they can rapidly raise $250,000 to $500,000 in Arkansas to launch his candidacy, and perhaps as much as $1 million by year's end.

"If he can get a little money behind him, everyone had better watch out because he's a whale of campaigner," said Sen. David H. Pryor (D-Ark.). "Right now he feels extremely confident about the response he's gotten."

Bumpers, a former Arkansas governor, refused to confirm or deny reports about his candidacy.

"I have no comment except to say I'll make an announcement shortly," he said before he left for speaking engagements with Democratic Party groups in Missouri and Oklahoma.

Traveling with him was political strategist David Doak, who reportedly has agreed to become his campaign manager. Doak managed Virginia Gov. Charles S. Robb's 1981 campaign and Texas Gov. Mark White's 1982 campaign. Both are Democrats.

Bumpers, a two-term senator, began serious consideration of entering the crowded Democratic presidential field after Sen. Edward M. Kennedy (D-Mass.) announced that he wouldn't make the race. Bumpers has visited 10 states since mid-January, impressing party regulars with his oratory.

Bumpers, according to sources, originally wanted to declare his candidacy and bypass forming an exploratory committee, but strategists persuaded him that he would attract more attention by delaying a formal declaration of candidacy until later.

His entry would expand the field for the Democratic nomination to seven: former vice president Walter F. Mondale, former Florida governor Reubin Askew, and Sens. Gary Hart of Colorado and Alan Cranston of California have already declared their candidacies; Sens. Ernest F. Hollings of South Carolina and John Glenn of Ohio are expected to announce in April.

Although highly regarded in the Senate, Bumpers would enter the race as one of the least known of the Democratic contenders.

His dark-horse candidacy is based on the idea that Democratic front-runner will falter in the early rounds of the race in 1984, opening it up for a fresh face.

In Arkansas, Bumpers has a reputation for being a giant-killer. In 1970, he pulled off one of the biggest upsets in state history by defeating Orval E. Faubus in the Democratic primary and the incumbent Republican governor, Winthrop Rockefeller, in the general election.

He was elected to the Senate four years later, defeating Vietnam war critic J. William Fulbright, a 30-year Senate veteran and chairman of the Foreign Relations Committee.

Bumpers is regarded as moderate on economic isssues and liberal on such social issues as busing for the purposes of school integration, school prayer and abortion. He has been an outspoken critic of President Reagan's defense buildup.