The battle for control of Washington this fall will again follow the Mason-Dixon Line.
The South - with the greatest number of House seats of any region - will prove crucial in deciding who will control Congress for the next two years.
The political tilt of the South - the region with the most electoral votes - will also help determine the 1984 presidential candidate in both parties, and the election itself.
Last week, however, the Southern conservative coalition went down to defeat on the $98.3 billion tax bill. This will likely lead the South to reappraise its position in national politics, analysts here say. Southern conservatives enjoyed a string of successes the first 18 months of the Reagan administration - ironically with the help of moderate Republicans who abandoned their own historical pattern and backed Mr. Reagan down the line. But those successes ran out in the 226-to-207 tax vote, at the hands of the familiar Yankee coalition of moderate Republicans and Northern Democrats that ruled Washington in the 1960s and 1970s.
Democrats say the key to regaining effective control of the House - even if they gain 15 more seats as generally predicted - will depend on whether they can build a coalition between Southern and Northern Democrats. Republicans admit their momentum in gaining Southern support has been put in question, if not slowed.
In short, the South, instead of proving itself the dominant American political force as many claimed after the 1980 election, appears to have returned to its historical pivotal position.
A Monitor analysis of last week's tax vote shows most strikingly a regional pattern. Southern congressmen voted 86 to 47 against the tax hike. Other regions voted the other way. Voting for the hike were the Midwest by 77 to 44, the East by 62 to 41, and the West by 41 to 35.
The analysis showed three other things:
* Despite their bipartisan victory, both the House Democratic leadership and the Reagan White House fared worse in maintaining party-line support than in previous key votes. House Speaker Thomas P. O'Neill Jr. (D) of Massachusetts held only 51 percent of House Democrats in line. This was well below the nearly 80 percent party discipline of previous key votes in this Congress, which compared favorably with the best Democratic discipline in previous administrations. Reagan and the Republican minority leadership held only 53.6 percent of GOP membership. This was well below the 99 percent plus GOP cohesiveness earlier this session, and below the average for the Eisenhower, Nixon, and Ford administrations.
* If it were not for the margin of votes provided by the nearly three dozen House members quitting their posts this fall, the vote could have failed. Congressmen not seeking reelection voted 20 to 12 in favor of the tax hike.
* There was a clear philosophical split in the tax vote, with the moderate-liberal group victorious over the conservatives. The 226 congressmen voting ''yes'' averaged 46 percent in their ideological ratings by the Americans for Democratic Action in 1981. Those voting ''no'' averaged a more conservative 33 percent.
* The safety of a congressman's seat was definitely a factor. Overall, both those voting for and against the tax measure averaged virtually the same 68 percent margin in their races for the House in 1980. However, congressmen in the 135 districts decided by 59 percent of the vote or less in 1980 - so-called ''marginal'' seats - tended to vote against the measure. Seventy-five of them voted no and 60 yes. This again showed the outcome might have been affected as much by reelection prospects as by ideology.
The vote last week showed some other interesting patterns. For instance, President Reagan's home state of California divided right down the middle, 22 yes and 21 no. Also, both party delegations split evenly - the Democrats 11 yes and 11 no, the Republicans 11 yes and 10 no. California, which historically offers a remarkably representative picture of American politics as a whole, showed little deflection from this role even with a Golden State native in the White House.
In the Midwest, where recession has hit deepest, supporting an economic recovery by voting for the tax hike - despite arguments that governments usually cut taxes in recession - appeared decisive. Midwest delegations voting for the tax bill included Michigan (13 to 5), Illinois (19 to 5), Ohio (14 to 10), Minnesota (5 to 3), and Wisconsin (7 to 2).
This pattern followed the industrial crescent from the Great Lakes into the East. Pennsylvania's delegation voted 14 to 11 for the tax hike, New Jersey's 9 to 6, and New York's by 20 to 13.
House leaders apparently had some success in some states - House Ways and Means chairman Dan Rostenkowski (D) of Illinois, for example - in persuading their state delegations to back the bipartisan bill. But others, notably House Budget Committee chairman Jim Jones (D) of Oklahoma, either failed to lead or went the other way - with Representative Jones joining the solid 6-to-0 Oklahoma delegation vote against the measure.
Ironically, congressmen from the safest, most liberal Democratic seats went both ways on the tax measure. Rep. Robert Garcia (D) of New York, who won with 98 percent of the vote in his district in 1980, compared with Reagan's 8 percent victory in his district, voted no. Rep. Charles Rangel (D) of New York, who won with 96 percent of his district to Reagan's 12 percent, voted with his party's leadership and the White House.
Overall, the latest House vote shows the leadership of Congress is still up for grabs. Democratic officials say they will gain a larger share of the South's seats in the November elections. They do not claim, however, that this will add up to effective Democratic rule of the House.
Looking ahead, many Democrats say a presidential candidate who can emphasize the conservative theme of ''prune, cut, and trim'' as well as emphasize Southern patriotism and strong America themes - possibly a candidate like Sen. John Glenn (D) of Ohio - would prove as effective in the South as a conservative Republican in 1984.
Reagan may be able to reassemble his winning coalition of boll-weevil Democrats and Northern Republicans to win future votes this session, party analysts agree. In fact, a motive behind Democratic support of the tax hike was the fear that Reagan would force them into a losing showdown for more spending cuts after Labor Day, while they wanted to get busy with their fall campaigns.
But the notion that the South's views are ascendant has been set back. The South must deal with the irony of having the largest delegation, but still being vulnerable to outvoting by the combined forces of the non-South.