In politics Lame Duck
is slang for a politician, who by legal restriction, electoral defeat or voluntary (and public) decision will not return to high office. The term is most generally used to describe an American President
late in his or her second term, whom the 22nd Amendment
prevents from seeking re-election to a third term. Lame Duck as a term can be used to describe elected officials whose term in office is pre-determined. Although the official retains all powers officially provided by the constitution
the official will suffer a slow and steady decline in real-world power as his or her term winds down.
The reason a lame duck's power declines is simple: they're going away! Politics is a contact sport, the mechanism by which disparate human beings resolve conflicting views about policy. So long as an elected official remains in power, he or she must be taken into account. Their support or opposition matters when when seeking legislation, or any other political matter within their purview. If you know President Gas is going to be in power for another four years, legislation must be made acceptable to him, or it probably will never become law. But if said President is leaving office in six months, it may be easier to wait out the term hoping for a more agreeable successor. At the end of his or her term, the President is almost irrelevant. The Chief Executive may propose and cajole all they wish, but the legislature (whose term will also expire) need only do nothing to kill any and all new initiatives.
A lame duck may matter more when both President and Congress are of the same political party, and have, or probably will be, voted out of office. In that case both legislature and President may be very active trying to pass legislation and executive decrees they know will not come to pass during the next term. They may be in an enormous hurry, but the opposition can and will use delaying tactics to block or undo many of these new initiatives. It's like at the end of a basketball game, where the clock is the friend of the team in front.
For this reason meaningful legislation is almost always passed during the beginning of a term of office. Only crisis or other external pressures lead dramatic action late during a term. The newly elected officials power is at its zenith. Popular officials, who retain the possibility and popularity required to return them to office retain their power throughout. Those who are leaving office lose power. Under other systems of government, this cycle of power peak and decline is not inherent. A popular politician in many systems can always call a new election, as Margaret Thatcher did shortly after Britain's victory in the conflict over the Falkland Islands (Malvinas). However no constitutional government is without its expiration date. Sooner or later popularity declines, and the party in power will end up as lame ducks.