The Democrats have a back-up plan.
If all else fails and Coakley loses, Democrats are prepared to delay credentialing Brown until after a final vote on a merged House-Senate health-care bill. The Senate majority has signaled it would not seat a victor in the special election without formal certification from Massachusetts’s top election official, Democrat William Galvin, a process that would give Senate Democrats at least ten days after the start of their 2010 legislative session to pass a bill with interim Senator Paul Kirk (D., Mass.) in place as a 60th vote.
Waiving rules is very unusual and very rare. Except when it's not:
But a by-the-book certification and seating would stand in contrast to Massachusetts’ last special election, in which Rep. Niki Tsongas (D., Mass.) was rushed to the House of Representatives on Oct. 18, 2007, just two days after she defeated Republican opponent Jim Ogonowski, before the state had certified her election. At that time, Democrats needed Tsongas to stand with them in overturning President Bush’s veto of the expansion of the State Children’s Health Insurance Program (S-CHIP.)
Senator Paul Kirk, you may remember was appointed, not elected. Let's not forgot how THAT all went down- from Wiki.
In 2004, the Massachusetts General Court had withdrawn the authority of the governor to fill a U.S. Senate vacancy by appointment, to prevent the then-Governor Mitt Romney, a Republican, from appointing a Republican to fill the remainder of Democrat John Kerry's Senate term, if Kerry were to win the 2004 presidential election. The legislation was enacted over Romney's veto. At that time, Senator Ted Kennedy successfully made personal appeals to Massachusetts Democratic legislative leaders to pass the bill, which had been stalled prior to his request. The new law called for a special election months later to fill the vacancy. However, Kennedy's death denied Democrats in the U.S. Senate the 60-vote supermajority required to end filibusters. Given the urgency of and narrow partisan support for some legislation before Congress, most notably health care reform, Democratic lawmakers and liberal pundits called for an interim senator to be appointed so that Massachusetts would have full Senate representation until the special election. In September, the General Court passed legislation restoring the governor's power to make interim appointments to serve until the special election stipulated in the earlier legislation is held, over multiple bipartisan concerns of hypocrisy