Washington - Veteran Republicans are bailing on Congress in growing numbers, as GOP control of Washington fails to produce the unity or legislative successes party leaders wish for.With US President Donald Trump willing, if not eager, to buck fellow Republicans and even directly attack them, a number of lawmakers no longer wish to be involved.The latest was two-term Representative Dave Trott of Michigan, who said in a statement on Monday that he'd decided after careful consideration that the best course for him was to spend more time with his family and return to the private sector.READ: Flake says GOP has let Trump divert from conservative idealsIn contrast to those diplomatic words was Trott's most recent tweet, sent in mid-August: "I think America needs more unity and less divisiveness... meaning @realDonaldTrump should focus more on golf & have less press conferences."Trott joins a string of moderate Republicans, including Reps Charlie Dent of Pennsylvania, Dave Reichert of Washington state and Ileana Ros-Lehtinen of Florida, who are not seeking re-election.Retirement rumoursEach of these seats will be heavily contested by Democrats eager to take back control of the House, and rumours abound of other GOP retirements still to come.New Jersey's Leonard Lance is weighing retirement, while another Michigan Republican, Rep Fred Upton, is mulling a campaign for US Senate, according to party operatives who spoke on condition of anonymity to discuss private deliberations.Also on Monday a senior GOP senator, Bob Corker of Tennessee, issued a statement indicating indecision about his future following a CNN report stating that the influential chair of the Foreign Relations Committee had not yet decided whether to seek re-election in 2018."It's not an automatic for me. It just isn't," Corker told reporters, although he added that as chair he has "a lot of impact without passing legislation. I can influence things. This is more about just what I believe to be the right thing to do."Although Republicans are hopeful Corker ultimately will decide to run - he already has $7.5m in his campaign account - the senator was in Trump's Twitter cross-hairs in August after criticising the president's response to the racially motivated protests in Charlottesville."Tennessee not happy!" the president declared after claiming that Corker was "constantly" asking him whether or not he should run again in 2018.The developments have alarmed GOP operatives concerned that the trickle of retirements could turn into a flood unless congressional Republicans and Trump can come together and produce on their promises, particularly by overhauling the tax code.And, with Trump bypassing Republicans to make deals with Democrats, and encouraging primary challenges against sitting GOP senators, the retirement decisions also reflect concerns among some about whether they will get party support when they need it, especially with former Trump adviser Steve Bannon threatening all-out war on congressional leadership."There are some stability concerns in the party about whose team everyone is on," said Josh Holmes, a GOP consultant and former chief of staff to Senate Majority Leader Mitch McConnell."Concerns about whether your party is really with you."DivisionsIt all illustrates that, far from producing unity within the Republican Party, the Trump era appears to be exacerbating existing GOP divisions while creating new ones.The familiar divide between pragmatic and ideologically driven Republicans has been heightened, while Trump's deal-making with top Democrats last week is forcing elected Republicans to choose sides between Trump and GOP leaders McConnell and House Speaker Paul Ryan."The party never united around Trump as it would another nominee, let alone president, and Trump is not a limited government conservative," said Alex Conant, a former top aide to Senator Marco Rubio."And so he is not a traditional Republican and as a result is going to clash with the traditional Republicans that fill the ranks of Congress."The chaos and uncertainty produced by Trump and his orbit would be more acceptable to congressional Republicans if the party was achieving legislative success.Instead, its long-standing promise to repeal and replace former president Barack Obama's health care law collapsed on the Senate floor in July, while other priorities are moving slowly.As a result, a number of Republicans on and off Capitol Hill have come to view tax reform of some kind as a must-pass priority, without which the dam would likely break on retirements and Republicans would be in serious jeopardy of losing control of the House."Republicans need to put points on the board, to deliver and show they are getting something done," said Tom Reynolds, a former New York congressman who once chaired the National Republican Congressional Committee and is now a lobbyist.Yet despite enthusiasm among Republicans, any final tax plan is a long way off, and many analysts are already predicting that Republicans will end up settling for some tax cuts that add to the deficit rather than full-blown reform.For their part, Democrats are projecting increased confidence about their prospects in next year's midterms, especially in the House, where they must gain 24 seats to win the majority.Republicans have a 240-194 edge, with one vacancy. Democrats have their highest hopes pinned on the 23 districts where GOP House candidates won in 2016, as did Democratic presidential nominee Hillary Clinton.House Minority Leader Nancy Pelosi pointed to Trump's overall approval rating nationally, which has dipped below 40%."There's probably nothing more dispositive of who wins next year's elections than where the president stands a year before," Pelosi told reporters."The year is fraught with meaning because that's when people decide whether to run or not, and that really is a timetable that's very important to us, and very positive for us right now."