One peculiar development of modem political systems finds place in the inter-penetration political parties and interest groups which is a two-party system characteristic of peculiarly English and American models or a multi-party system. This inter-action and inter-penetration is existent even in a communist society where trade unions and other cultural organisations work in unison with the only dominant political party. What counts in politics is power. For the sake of having power both the parties and the organised groups have to depend upon the help and cooperation of each other, Hence, either some groups combine together to form or support a particular party as a political counterpart of their organisation or coalition, or a political party controls some group or groups and thereby imparts a politico ideological content to their activity. The result is not only a process of interaction but interpenetration: when the party controls groups, it inhibits the capacity of groups to formulate pragmatic specific demands, but when the groups control a party, they inhibit the capacity of a party to combine specific interests into programmes with wider appeal.
It follows that in a political system organized interest groups not only constitute necessary elements, they also playa very important part in the political process as a whole. The process of interaction and interpenetration between the two is so strong that to account for the directions and control of public policy, both must be taken into the scope of study.
Even if a group moves from its advocacy of a particular narrow interest to the whole range of public questions, it endeavours to stick, as far as possible, to the facade of nonpartisanism and thus its alliances of opinions and attitudes are supplemented by relations of mutual defence and offence. Sometimes, a group may adhere to a particular line of thinking or action and thereby create a point of sharp difference with a similar group and come very close to a particular political party. Such a situation creates a degree of collaboration between the party and the group where the latter has to wind itself up generally on one side of the fence. The result is, as Key suggests, “not necessarily party adoption of the group programme, it may be modified in the light of the broader objectives of the party as a whole.”Political parties and pressure groups resemble each other in a way that both seek to realise their objectives by influencing the decision-making agencies in their favour .
However, a striking difference between the two finds place in that while a party is a fullfledged political organisation and plays politics by profession, a group becomes a political association for a particular purpose only to play politics for the sake of expediency. A group is formed just for the sake of some interest to exercise influence upon the decision-making body to the extent its range of interest is involved. Very often a pressure group lives and acts in a most disguised manner and strives for its political promotion according to the ethos of its existence That is, a group keeps or unkeeps its political complexion just for the sake of expediency. It is thus very rightly pointed out by Harry Eckstein that pressure group politics “represents something less than the full politicisation of groups and something more than utter depoliticalisation; it constitutes an intermediate level of activity between the political and the apolitical.
• A group may pursue its objective in dealing with governmental agencies almost independently of a political party. That is. by virtue of its privileged position and favourable circumstances, it may have a mechanism of communications with the governmental functionaries autonomous of party control.
• A group may seek to realise its goal by means of active collaboration with a single political party.
• A group may neither be so powerful as to work independently of party control nor so conservative as to live invariably under the hold of a single political party. Thus without having a formal mask of partisan orientation it may have affiliations with any party or parties as per its own interest. It is due to this reason that on the national scene certain major pressure groups / cluster around each party. The policy orientation of the parties plough a furrow through the group system, and most of the major groups – and some of the lesser groups – find one party far more congenial to their tastes than the other.
• A political party is pledged to have a partisan orientation and this factor contributes to the inelastic character of a party organisation. But with pressure groups the case is entirely different.