A second argument is that impeachment could never happen because even if Bush and Cheney were removed, it would mean Nancy Pelosi would become president, and Republicans would never allow this to happen.
Some who raise these issues may be genuinely horrified at the prospect of a Cheney presidency (though we really already have that), and may also be genuine in thinking that there could never be an impeachement that would change control of the White House from one party to the other. But I get the feeling that many of those who raise these objections to impeachment simply don't want to deal with impeaching the president. Certainly the Cheney canard has been quietly raised by many in the Democratic leadership to explain their shameful inaction in defense of the Constitution in the face of Bush's many grave crimes and abuses of power.
So let me knock these two bogus concerns down once and for all.
First Cheney. There is a precedent here. Richard Nixon, as he faced impeachment, also had what many said was a kind of impeachment insurance: Spiro Agnew. Agnew was in some ways like Cheney--a hard-right, money-grubbing, small-minded proto-fascist. He was also different in that Cheney is embedded in the highest reaches of the corporate petro-war machine, while Agnew was a political light-weight. Cheney also seems to have a Machevellian cleverness that Agnew never had. That said, both have in common that they are profoundly disliked by the vast majority of Americans--Cheney even more so than Agnew, who at least had a robust following among the yahoo, know-nothing crowd (that's pre-Yahoo yahoo usage).
In any event, when it became clear that Nixon was going to go down, Republican Party leaders looked aghast at the prospect of facing the voters with Agnew as the face of the Republican Party. They found a way out, by having the Justice Department indict Agnew on bribery charges. He was gone in a flash, and thus they were able, with Democratic Congressional support, to put a safe, uncontroversial place-holder into the vice president's office, Gerald Ford, who was at the time the minority leader of the House. It made a certain sense given that had Republicans been in control of the House, Ford would have been the next in line for the presidency.
If impeachment hearings made it clear that Bush's days in office were numbered, as I believe would certainly be the case, Republicans would be at least as concerned about being stuck with Cheney as their party leader heading into 2008 as their forebears were of having Agnew in that position. I have no doubt but that they would push him or threaten him or drive him out of office in the same way that they eliminated Agnew. They'd have several ways to do that. Cheney, who has myriad health problems, including a bionic heart and thrombosis, could simply push a little emergency escape button on his life-support system and claim he was having some heart problems and had to leave for health reasons. If he didn't do that, there is no doubt a file lodged in the FBI somewhere with enough serious dirt on Cheney's financial chicanery to pull him down with an indictment. He knows that, and so would almost certainly cut a deal that would allow him to skate away free. In any event, he'd be gone.
In the unlikely event that he chose to ignore the pressure, and managed to intimidate his party colleagues and the attorney general's office into not indicting him, the other problem for Cheney is that he is so intricately involved in all of Bush's impeachable offenses that he would surely be impeached along with the president or more likely indicted by a special prosecutor long before the president resigned or was thrown out of office.
As for Pelosi becoming president, I think it is impossible for two reasons. Firstly, if Democrats ever develop the courage and sense of principle to initiate impeachment hearings into Bush's crimes, they, like their predecessors in 1974, will want to gain at least some Republican backing, and they could never do this if Republicans thought they might be handing the White House over early to them. Clearly they would want to make it plain that they had no intention of installing Pelosi in the White House via impeachment. I suspect that the same kind of arrangement would be made in Bush's case as was made in Nixon's: Republicans would be able to pick an uncontroversial, lackluster replacement for Cheney--probably someone like House Minority Leader John Boehner (R-OH)--to take over from Cheney. (Boehner is in many ways a Gerald Ford clone--undistinguished politically and intellectually, and unlikely to win should he run for president on his own right. There is also the delightful way that his official bio is so quick to explain that his name is pronounced "Bay-nor," and not the way one might expect it to be pronounced, which would be certain to make him the butt of late-night TV jokes the same way Ford was.)
The main thing is that it is simply not correct that the fear of a Cheney or a Pelosi presidency provides Bush with some kind of insurance against impeachment.
The only insurance Bush has against the impeachment he so richly deserves, and that a majority of Americans devoutly wish to see him receive, is a craven Democratic Party leadership, which because of a profound lack of principle, an excess of self-interested political calculation, and an astonishing misreading of the popular will, is going to any lengths to avoid doing what the Constitution demands it to do: impeach a president who poses a clear and present danger to the survival of Constitutional government and the rule of law in America.