Democratic Party officials, buoyed by the impending retirement of Sen. John G. Tower (R-Tex.), are increasingly confident of regaining control of the Senate in next year's elections, while White House political advisers are warning President Reagan privately that if he seeks reelection he cannot count on a Republican Senate in 1985.

Even before Tower's surprise decision was announced on Tuesday, the race for the Senate was shaping up as a contest equally important as the race for the White House, according to leaders of both parties. Tower's retirement does not alter either party's chances next year significantly, but it has quickly focused public attention on the importance of that the battle for the Senate.

"The fundamental difference this time is that it is well known in advance that control of the Senate is at stake," said Mitchell E. Daniels Jr., executive director of the National Republican Senatorial Committee.

The Democrats must win five seats next year to regain control of the Senate, which the Republicans captured in Reagan's landslide of 1980.

Democratic Party officials say they believe they already have an excellent chance of picking up seats in Tennessee, North Carolina, Iowa and Texas. In addition, they are looking closely at races in a number of other states where they believe Republican incumbents are highly vulnerable.

White House political advisers agree that Republican prospects are grim, although White House spokesman Larry Speakes said yesterday that "We are not conceding a single seat in the Senate."

Speakes' comment came a day after a White House official, speaking on background, said that Sen. Jesse Helms (R-N.C.), a leader of the New Right, was "gone" in his effort to win reelection and predicted difficult battles for Sens. Strom Thurmond of (R-S.C.), Pete V. Domenici (R-N.M.), Charles H. Percy (R-Ill.), Gordon J. Humphrey (R-N.H.) and Roger W. Jepsen (R-Iowa). Both parties have been gearing up for months, raising money, recruiting candidates and, wherever possible, softening up incumbents.

The National Republican Senatorial Committee raised a record $15 million in the first six months of 1984 with appeals to save the Republican Senate. Committee Chairman Richard G. Lugar (R-Ind.), concerned about the party's image with women, is attempting to persuade several women to challenge Democratic incumbents--so far without success.

Meanwhile the Democratic Party, under the leadership of Sen. Lloyd Bentsen (D-Tex.), who heads the Democratic Senatorial Campaign Committee, has been more aggressive than ever in recruiting candidates and raising money, and has tried to stir up problems for Republican incumbents in their home states. It has raised only $1.5 million, however, one-tenth the Republicans' amount.

"This has been looked on as an incumbents' committee," Bentsen said recently. "I've changed that."

The Democrats also say they believe they can capitalize on the "gender gap" in Senate races and are counting on an increase in black voter registration to help them, especially in several southern states with key races.

After this opening round of activity, it is the Democrats who sound the most confident.

"It may only be the first quarter of the ball game, but right now we're pleased with where we stand," Brian Atwood, executive director of the Democratic Senatorial Campaign Committee, said yesterday.

Republicans are more defensive, publicly and privately.

"I think the Democrats are not going to win the Senate , Lugar said. "So I think their chances are less than 50-50."

The numbers appear to favor the Democrats, however, as they did the Republicans in the last two elections.

Of the 33 senators up for reelection, 19 are Republicans and 14 are Democrats.

Two of those Republicans and one Democrat have chosen not to run. Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) announced earlier this year that he would retire at the end of his term, and Tower joined him with his surprise announcement on Tuesday.

Sen. Jennings Randolph (D-W.Va.) announced his retirement last March but Gov. John D. Rockefeller IV is heavily favored to keep his seat in the Democratic column.

Rep. Albert Gore Jr. (D-Tenn.) is considered the favorite to win Baker's seat next year, and the Republicans are having problems finding an attractive candidate to run against him.

Tower's retirement will give Texas Democrats added impetus to try to win that seat.

Based on the state party's performance in 1982, when it knocked out Gov. Bill Clements (R) and elected a slate of progressive Democrats by landslide margins, they are in a good position to do so. But Daniels said yesterday that Reagan's popularity and the prospect of a weak field of Democrats made him confident of holding onto the Texas seat.

The Senate seat the Democrats most want to win is in North Carolina, where Helms faces a well-financed, popular challenger in Gov. James B. Hunt (D).

"It's a national contest," Lugar said recently. "It's a large political war. Both are attracting monumental financial support and personnel support. It has dynamics of its own."

The latest polls give Hunt a lead of almost 20 points, but both sides admit that it is far too early in this high-visibility race to make predictions.

Helms already has raised $1.7 million and spent $1.6 million on the race, and for several months has been attacking Hunt with a barrage of radio and newspaper ads.

Democrats also believe Rep. Tom Harkin (D-Iowa) has an excellent chance of defeating Jepsen, whose image has been tarnished recently by a series of well-publicized problems. The latest Iowa Poll shows Harkin leading Jepsen by 5 percentage points.

The Democrats' one concern about the race, however, is about Harkin's ability to raise money to take on an incumbent who is expected to have a sizable campaign treasury.

The other race that is drawing attention is in Illinois, where Percy, chairman of the Foreign Relations Committee, is being challenged in the primary by conservative Rep. Tom Corcoran and where at least three Democrats are seeking the nomination.

Percy's office denied rumors this week that the three-term incumbent, who had a scare in his 1978 race, might yet follow Baker and Tower into retirement. They said Percy already is campaigning actively.

Democratic officials recruited Rep. Paul Simon (D-Ill.) to make the race. The other Democratic candidates are Alex Seith, a Chicago attorney who lost to Percy in 1978, and Illinois Comptroller Roland Burris, who is black. Democratic state Chairman and Senate President Philip Rock has said that he also intends to run.

In Mississippi, Democratic officials are continuing to court outgoing Gov. William F. Winter to run against Sen. Thad Cochran (R). A poll taken by the Democrats showed Winter leading Cochran.

In New Hampshire, Republican Humphrey faces a challenge from Rep. Norman E. D'Amours (D-N.H.). Democrats avoided a potentially bitter primary when former senator John Durkin decided to run for D'Amours' House seat.

Democrats also say Sen. Rudy Boschwitz (R-Minn.) is vulnerable. So far two potential Democratic challengers have emerged, Rep. James L. Oberstar and Secretary of State Joan Growe.

Maine Democrats are still trying to persuade Gov. Joseph E. Brennan, whom they consider their strongest candidate, to run against Sen. William S. Cohen (R-Maine).

In South Carolina, a recent newspaper poll showed Thurmond losing to Gov. Richard W. Riley. Riley says he will not enter the race, but the Democratic campaign committee is using the poll to try to recruit a challenger.